Magnetic recording medium

ABSTRACT

A magnetic recording medium is disclosed, comprising a non-magnetic support having provided thereon at least a lower non-magnetic layer comprising a binder having dispersed therein a non-magnetic powder and an upper magnetic layer comprising a binder having dispersed therein a ferromagnetic powder which has been coated on said lower non-magnetic layer while the lower non-magnetic layer is wet, wherein the upper magnetic layer has an average dry thickness (d) of not more than 1.0 μm and an average thickness variation (.sup.Δ d) at the interface between the upper magnetic layer and lower non-magnetic layer is not more than d/2. The magnetic recording medium exhibits excellent electromagnetic characteristics, running properties, and durability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation of U.S. patent application Ser. No.08/846,035, filed Apr. 25, 1997, for MAGNETIC RECORDING MEDIUM, now U.S.Pat. No. 6,025,082 which is a Continuation of U.S. patent applicationSer. No. 07/873,201, filed Apr. 24, 1992 which is now U.S. Pat. No.5,645,917, which is a Continuation-In-Part Application of U.S. patentapplication Ser. No. 07/822,975 filed Jan. 21, 1992, now U.S. Pat. No.5,258,223.

FIELD OF THE INVENTION

This invention relates to a magnetic recording medium, and moreparticularly to a thin high-density magnetic recording medium having amagnetic layer of not more than 1.0 μm in thickness which exhibitsexcellent electromagnetic characteristics, running properties,durability, and satisfactory productivity.

BACKGROUND OF THE INVENTION

Magnetic recording media comprising a non-magnetic support havingthereon a magnetic layer comprising a binder having dispersed therein amagnetic powder, such as ferromagnetic iron oxide powder, Co-doped ironoxide powder, CrO₂ powder, and a ferromagnetic alloy powder havewidespread applications as video tape, audio tape, and magnetic discs.

Short wave recording has recently been introduced to meet the demand foran increased recording density. For example, the recording wavelengthfor 8 mm-video tape has reached 0.54 mμ. With this tendency, there hasarisen a problem of so-called thickness loss on reproduction, that is,reproduction output is reduced as a function of increasing magneticlayer thickness.

In order to cope with this problem encountered in short wave recording,magnetic recording media using a thin film of a ferromagnetic metal havebeen put to practical use which have a very small thickness due to useof formation methods such as vacuum deposition techniques. Suchmetal-deposited recording media suffer little thickness loss and attaina very high reproduction output. However, production of such thin filmmagnetic metal-deposited recording media by vacuum evaporation of ametal on a non-magnetic support is less suited to mass production ascompared with so-called coated type magnetic layers formed by theconventional coating techniques involving dispersions of ferromagneticpowders in a binder system. In addition, the metallic film is lessreliable for long-term use because of susceptibility to air oxidation.

Therefore, it has been attempted to instead reduce the thickness of amagnetic layer formed by various manipulations of the conventionalcoating technique to thereby increase reproduction output. However, asthe thickness of a magnetic layer is decreased to about 2 μm or less,the surface properties of a support are apt to strongly influence thesurface properties of the magnetic layer resulting in deterioration ofelectromagnetic characteristics.

In order to reduce the thickness of a magnetic layer to minimizethickness loss of magnetic properties thereby achieving a high outputwhile excluding the adverse influences of a support surface, it has beenproposed to provide a thick non-magnetic layer between a non-magneticsupport and a thin magnetic layer. For example, U.S. Pat. No. 2,819,186discloses a magnetic recording medium comprising a support havingthereon a hard and brittle magnetic layer having a magnetic substancecontent of 85% by weight or more and a thickness of not more than 0.25mil as an upper layer and a soft and flexible non-magnetic lower layerhaving a higher thickness than the upper magnetic layer. JP-A-62-154225(the term "JP-A" as used herein means an "unexamined published Japanesepatent application") discloses a magnetic recording medium having amagnetic layer thickness of 0.5 μm or less with a subbing layer providedbetween the magnetic layer and the support containing carbon black as aconductive fine powder and having a thickness greater than the magneticlayer so as to prevent surface resistance of the magnetic layer fromincreasing. JP-A-62-222427 discloses a magnetic recording mediumcomprising a support having thereon a subbing layer containing anabrasive having an average particle size of from 0.5 to 3 μm and a 1 μmor less thick magnetic layer containing a ferromagnetic powder, in thisorder, in which a part of the abrasive in the subbing layer projectsthrough the magnetic layer so as to serve for cleaning of a magnetichead. Thus, it has been suggested to provide a non-magnetic lower layerimmediately adjacent the support to reduce the thickness of a magneticlayer thereby to achieve high-density recording and, at the same time,to incorporate into the lower non-magnetic layer additives such ascarbon black for static charge prevention or an abrasive for improvementin cleaning characteristics or durability.

However, conventional techniques for producing these magnetic recordingmedia having a lower non-magnetic layer and an upper magnetic layerinvolve complicated processes. For example, such processes comprisefirst coating a non-magnetic layer on a non-magnetic support, thendrying the non-magnetic layer, and then, if desired, followed bycalendering, and thereafter coating a magnetic layer thereon. However,problems have been identified with these conventional techniques.

For instance, reduction in thickness of a magnetic layer is achievedeither by reducing the application rate or by using an increased amountof a solvent in a magnetic binder coating composition to reduce theultimate film concentration. Yet, when the former approach is taken,drying of the coating transpires too quickly before sufficient levelingcan occur to leave surface defects such as streaks or traces of coatingpattern, resulting in very poor yield. On the other hand, when thelatter approach is taken, a thin coating composition provides a coatingfilm with many voids, resulting in shortage of packing of aferromagnetic powder or insufficient film strength.

In order to overcome these problems, it has been proposed to form anon-magnetic layer as a lower layer and a thin coat of a highlyconcentrated magnetic coating composition by a simultaneous coatingsystem. For example, JP-A-63-191315 discloses a magnetic recordingmedium having a lower non-magnetic layer and an upper magnetic layerformed by simultaneous coating, in which the lower layer has a thicknessof 0.5 μm or more and contains no polyisocyanate.

Extensive studies have hitherto been given to the above-describedsimultaneous coating system or successive wet coating system, calledwet-on-wet coating, for formation of a plurality of magnetic layers.However, the same techniques cannot be applied to the lower non-magneticlayer to obtain satisfactory results. That is, where a lowernon-magnetic layer and an upper magnetic layer are formed by wet-on-wetcoating, disturbances occur in the interface between the upper and lowerlayers, causing pinholes or run-away of the magnetic coatingcomposition.

Further, although a thick non-magnetic layer formed beneath a magneticlayer eliminates the influences of the surface roughness of the support,the problem of wearability against a recording head or durability isleft unmitigated. The poor wearability or durability of conventionalmagnetic recording media having a non-magnetic lower layer appearsattributable to curing of the lower layer comprising a thermosettingresin as a binder for reasons that the magnetic layer formed thereon isbrought into contact with a head or other members without cushioning andthat the magnetic recording media having such a cured lower layer lacksthe desired flexibility.

This problem might be resolved by using a non-curing (thermoplastic)resin as a binder in the lower layer. However, when a magnetic layer iscoated on a dry lower layer containing such a non-curing resin, as inthe conventional technique, the lower layer is swollen with the organicsolvent of the magnetic coating composition, giving undesiredinfluences, such as turbulence of the magnetic coating composition,leading to impairment of the surface properties of the magnetic layerand deterioration of electromagnetic characteristics.

Furthermore, a magnetic coating composition must be diluted with arelatively large quantity of a solvent before it can be coated to a drythickness of not more than 1.0 μm. Such a diluted coating composition issusceptible to agglomeration. Further, orientation of a ferromagneticpowder is apt to be disturbed during drying due to evaporation of thelarge quantity of organic solvent. When the medium has a non-continuousform, for example, if it is a magnetic disc, adequate performanceproperties may still be obtainable to some extent even in using such athin magnetic coating composition. However, with respect to those mediahaving a continuous form, such as magnetic tapes, although the purposeof thickness reduction is accomplished, it is difficult to obtainsufficient electromagnetic characteristics because of deterioratedorientation and deteriorated surface properties. In addition, many voidsare produced during drying, resulting in poor film strength in running.If the amount of the diluting organic solvent is decreased in order toimprove orientation properties and to minimize voids, the coatingstability would be deteriorated, leading to formation of many pinholesand an increased production of defective media.

On the other hand, it is known that performance properties of digitalrecording media may be improved by reducing the thickness of a magneticlayer. Thickness reduction is effective, in principle, but gives rise toproduction problems. That is, coating defects such as pinholes andcoating streaks occur, and a sufficient yield cannot be reached.Further, since calendering effects would be reduced with thicknessreduction, the resulting magnetic layer has poor surface properties andunsatisfactory electromagnetic characteristics.

It is suggested to overcome these problems by simultaneously forming arelatively thick non-magnetic layer as a lower layer and a thin magneticlayer of 1 μm or less in thickness as an upper layer, followed bycalendering. To this effect, incorporation of non-magnetic abrasiveparticles or fillers into the lower layer has been proposed as disclosedin JP-A-62-22242 and JP-A-2-257424. However, when a magnetic layer and anon-magnetic layer are simultaneously coated, and the magnetic substancein the upper layer is orientated, the two layers are mixed at theinterface due to the rotary motion of the magnetic substance in amagnetic field. As a result, the surface properties and orientationbecome insufficient, with a failure to obtain sufficient electromagneticcharacteristics.

It has been proposed to provide a non-magnetic and conductiveintermediate layer containing graphite flakes to improve orientation ofthe magnetic powder in the upper layer as described in JP-A-55-55438.Although an improvement in orientation can be achieved by this proposal,graphite itself has no film reinforcing effect, and the resultingrecording medium lacks in durability. Incorporation of an inorganicpowder having a Mohs hardness of 5 or more into the non-magnetic layerhas also been proposed as disclosed in JP-A-60-125926. Similarly, it hasbeen proposed to provide a non-magnetic reinforcing layer containingacicular oxalate particles to improve orientation of the magnetic powderin the upper layer as disclosed in JP-B-58-51327 (the term "JP-B" asused herein means an "examined published Japanese patent application").

Orientation properties and durability can be improved by theseproposals. In actual production of magnetic recording media,nevertheless, both the flaky particles and oxalates impair surfacesmoothness of the magnetic layer because the former is susceptible toparticle stacking, and the latter exhibits poor dispersibility inbinders.

Further, in order to achieve high-density and high-output recording,magnetic recording media are demanded to have high surface smoothness soas to minimize spacing loss in contact with a recording head.Accordingly, a lower non-magnetic layer, while not being exposed, isalso increasingly demanded to have a smooth surface for coating an uppermagnetic layer thereon. In addition, the influence of dispersibility inthe lower non-magnetic layer on the surface properties of the uppermagnetic layer simultaneously formed thereon increases with thethickness reduction of the magnetic layer. Further investigationsrevealed that only an improvement in dispersibility of the lower layerdoes not suffice for obtaining satisfactory surface smoothness of theupper layer which is simultaneously formed thereon.

A magnetic layer should have a considerable coercive force (Hc) becauseif a magnetic layer has a low coercive force, it suffers a greatself-demagnetization loss and is not suitable for short wave recording.To this effect, it has been proposed to provide a subbing layer having athickness of from 0.5 to 5.0 μm between a non-magnetic support and amagnetic layer so that the magnetic layer may have an Hc of 1000 Oe asdisclosed in JP-A-57-198536.

However, conventional techniques when applied to effecting this proposalinvolve problems. That is, when the technique disclosed inJP-A-57-198536 is used for simultaneous formation of such a subbinglayer and the upper magnetic layer, the upper and lower layers aremixed, causing not only deteriorated surface properties but disturbedorientation. A technique for improving orientation in simultaneouscoating is suggested in JP-A-3-49032, in which carbon black is dispersedin the lower layer, and orientation is conducted in multiple stages.Nevertheless, fillers having a small true specific gravity such ascarbon black yield to the influence of the rotary motion of the magneticsubstance during orientation, resulting in disturbance of the interfacebetween the upper and lower layers on simultaneous coating. Thus, theabove proposal, though achieving a high squareness ratio as measured inthe planar direction, was insufficient for obtaining an improvedresidual coercive force in the direction of the normal of the magneticlayer as purposed.

An approach proposed to be taken to cope with these problems isdisclosed in JP-A-62-1115. However, when the technique disclosed isapplied to simultaneous coating as adopted in the present invention, thefollowing problems arise. That is, where carbon black of low specificgravity is used in the non-magnetic lower layer, simultaneous coating orthe subsequent orientation induces mixing of the non-magnetic lowerlayer and the upper magnetic layer or interfacial disturbances due toturbulence. Such mixing or disturbance at the interface extremelyreduces orientation properties of the magnetic substance in the magneticlayer.

In case of using magnetic particles having a short major axis and asmall acicular ratio, which are essentially insusceptible to floworientation, reduction of orientation properties is conspicuous with afailure to obtain sufficient electromagnetic characteristics.

In recent years, magnetic powders to be used in a magnetic layer havebeen reduced in size to meet the demand for high-density recording. Asthe particle size decreases, the strength of the magnetic layer is soreduced. It follows, for example, the tape is stretched under hightension during preparation or running on a video deck to have increasedskewness. Countermeasures against this include reduction of percentthermal shrinkage of the support or strengthening of the support, butthe effect obtained is limited. Further, when a simultaneous coatingsystem is adopted, the percent thermal shrinkage becomes greater ascompared with that in the case of a successive coating system(wet-on-dry), resulting in an increase of skewness. This is because, inthe latter case the lower layer is hardened after being coated bycalendering or curing so that the medium is prevented from stretching,while in the former case in which the upper and lower layers are coatedat once, stretching of the medium cannot be suppressed by the lowerlayer. Thus, the techniques disclosed in JP-A-63-187418 andJP-A-63-191315 exploiting a simultaneous coating system are accompaniedby these disadvantages.

A tendency of reducing tape thickness is also developed in an attempt toextend the time of playing. Reduction in tape thickness leads toreduction in tape stiffness and, as a result, satisfactory contact witha head is impaired, resulting in reductions in electromagneticcharacteristics. In particular, currently spread long-playing tapes for8 mm-VTR or VHS have a total thickness of not more than 14 μm and have adifficulty in assuring satisfactory contact with a head. Withconventional thick tapes, it has been rather effective for maintenanceof smooth contact with a head to reduce the strength of the lowernon-magnetic layer, but the latest thin tapes used in recording andreproducing apparatus using a rotating head can hardly obtain goodcontact with a head unless the stiffness of the lower non-magnetic layeris increased. Use of a stretched non-magnetic support might be effectiveto control the lower layer stiffness but causes a reduction in stiffnessin the width direction, which is unfavorable for running durability.

As described in JP-A-63-191315, although use of no polyisocyanate in alower non-magnetic layer is recognized effective for improving contactwith a head, such a medium turned out to be inferior in preservabilityunder a high temperature and high humidity condition. Although effectivein systems attaching no weight to preservability, this technique isunsuitable to systems demanding preservability, for example, forbusiness use or for data preservation. JP-A-63-187418 also disclosesthickness reduction of a magnetic layer for improving electromagneticcharacteristics, but the electromagnetic characteristics attained werestill unsatisfactory. JP-A-50-803 teaches use of non-magnetic pigmentfine granules having a Mohs hardness of at least 6 between a magneticlayer and a support. This proposal chiefly aims at polishing of analuminum support with a non-magnetic powder having a Mohs hardness of 6or more to thereby increase flatness of the support.

Hence, the techniques so far developed are incapable of satisfying thedemand of reducing thickness of magnetic recording media to cope withthe recent trends to extension of play time and high-density recording.That is, the conventional techniques could not achieve sufficientconsistency between excellent electromagnetic characteristics andrunning durability. In particular, to improve running durability whilereducing tape thickness requires minimization of tape edge damages. Fromthis viewpoint, the techniques of JP-A-63-191315 and JP-A-63-187418 areinsufficient.

Various proposals have ever been made for obtaining magnetic recordingmedia having an upper magnetic layer and a lower non-magnetic layer bywet-on-wet coating. For example, JP-A-50-104003 implies wet-on-wetcoating but shows use of only carbon black as a non-magnetic layer, inwhich the layers suffer from serious interfacial disturbance due to toostrong structural viscosity.

JP-A-62-212922 (corresponding to U.S. Pat. No. 4,916,024) discloses amagnetic recording medium having a conductive intermediate layercontaining carbon black and a ferromagnetic powder in a proportion offrom 5 to 25% by weight based on carbon black. The ferromagnetic powderis used for the purpose of improving dispersibility of carbon black.However, since the ferromagnetic powder used in the intermediate layerhas equal magnetic properties, the interfacial disturbance cannot besatisfactorily prevented. JP-A-62-214524 discloses a process forproducing a magnetic recording medium, in which a plurality of layersare wet-on-wet coated. This technique is characterized by selection ofthe formulation of each coating composition so that the solvent andsolute in each layer exhibit mutual solubility with those of theadjacent layer. Combinations of an upper magnetic layer and a lowernon-magnetic layer are illustrated therein, but the examples givenrelate only to selection of binders. Incorporation of carbon black isalso suggested but failed to eliminate the interfacial disturbance.JP-A-62-241130 (corresponding to U.S. Pat. No. 4,839,225) discloses amagnetic recording medium in which the intermediate layer contains atleast one binder carrying a hydroxyl group and/or an amino group and themagnetic layer contains an isocyanate compound. This technique aims atchemical bonding of the specific binder and the isocyanate compound tothereby bring about an improvement in adhesion strength between the twolayers. It is described that the intermediate layer may contain carbonblack, and the layers may be coated by a wet-on-wet coating system.However, the problem of interfacial disturbance could not be resolved bysuch disclosures.

On the other hand, JP-A-63-88080 (corresponding to U.S. Pat. No.4,854,262) discloses a coating apparatus having an improved doctor edge.The disclosure refers to a viscosity at a high shear rate (10⁴ sec⁻¹)but only showing the viscosities of coating compositions for the upperand lower layers. Such a disclosure fails to sufficiently inhibit theinterfacial disturbance.

JP-A-63-146210 discloses a magnetic recording medium in which the lowermagnetic layer or non-magnetic layer contains a non-curing binder andthe uppermost magnetic layer contains an electron-curing binder resin.However, the illustrated lower non-magnetic layers are only thosecontaining carbon black, and the interfacial disturbance was stillunmitigated. JP-A-63-164022 discloses a method for coating a magneticcoating composition, in which multiple layers are extrusion coatedthrough a slot die with a magnetic coating composition having a highdensity being sandwiched in between nonmagnetic coating compositionshaving a viscosity lower than that of the magnetic coating compositionthereby to improve high-speed thin coating properties. The interfacialdisturbance could not be sufficiently eliminated by this technique.JP-A-63-187418 (corresponding to U.S. Pat. No. 4,863,793) discloses amagnetic recording medium in which the ferromagnetic powder of the uppermagnetic layer has an average major axis length of less than 0.30 μm asmeasured with a transmission type electron microscope and a crystallitesize of less than 300 Å as measured by X-ray diffractometry. Thedisclosure includes incorporation of carbon black, graphite, titaniumoxide, etc. into the lower non-magnetic layer, referring to a specificcombination of 100 parts by weight of α-Fe₂ O₃ and 10 parts by weight ofconductive carbon. However, the amount of carbon used is small, and theparticle size of α-Fe₂ O₃ is not specified, and the interfacialdisturbance could not be sufficiently settled.

JP-A-63-191315 (corresponding to U.S. Pat. No. 4,963,433) discloses amagnetic recording medium in which the lower layer contains athermoplastic binder and has a dry thickness of 0.5 μm or greater,specifically illustrating a combination of 100 parts by weight of α-Fe₂O₃ and 10 parts by weight of conductive carbon similar toJP-A-63-187418. However, the amount of carbon used is small, and theparticle size of α-Fe₂ O₃ is not specified, and the interfacialdisturbance could not be sufficiently settled.

JP-A-2-254621 discloses a magnetic recording medium, in which anon-magnetic layer mainly comprising carbon black is provided, and amagnetic layer containing Fe--Al ferromagnetic powder is wet-on-wetcoated thereon. However, the illustrated example of the lower layercomprises only carbon black, which has too a high structural viscosityto remove the interfacial disturbance.

JP-A-2-257424 discloses a magnetic recording medium in which thenon-magnetic layer contains a filler having an average particle size of50 μm or greater Carbon black and abrasives, e.g., Al₂ O₃ and SiC, aregiven as examples of useful fillers. However, the specificallyillustrated non-magnetic layer contains carbon black alone, Al₂ O₃alone, or SiC alone. The problem of interfacial disturbance could not beresolved with such a combination.

JP-A-2-257425 discloses a magnetic recording medium containing aplurality of layers each having a coefficient of dynamic friction of notmore than 0.25 and a surface specific resistivity of not more than1.0×10⁹ Ω/sq. Illustrative examples of the non-magnetic powder to beadded to the lower layer are limited to SnO₂ alone and carbon blackalone, and the interfacial disturbance could not be avoided.JP-α-2-260231 discloses a magnetic recording medium comprising anon-magnetic support having laminated thereon a first non-magneticlayer, a first magnetic layer, a second non-magnetic layer, and a secondmagnetic layer in this order. The illustrated non-magnetic layers solelycomprise binders, failing to remove the interfacial disturbance.

JP-A-3-49032 (corresponding to U.S. Pat. No. 5,051,291) discloses amagnetic recording medium whose magnetic layer has a thickness of notmore than 1.5 μm and a squareness ratio of not less than 0.85. While anincreased squareness ratio can be obtained through multiple stageorientation, the lower layer contains only carbon black and failed toeliminate the interfacial disturbance due to its too strong structuralviscosity.

In recent years, Hi 8 tapes have been given studies, ultimately seekingfor obtaining merits possessed by both ME (vacuum deposited) tapes andMP (metal) tapes. It has been the most important subject to achieve sucha high C/N in the short wavelength region (luminance signals in highregion) as reached by ME tapes by using MP tapes while maintaining theexcellent performance properties possessed by MP tapes, i.e., runningproperties, durability, and production suitability.

In seeking for improvements in performance of video tapes, attention hasbeen accorded to the signal recording mechanism of VTR, i.e., recordingdepth of each signal, and double coating technique has been manipulatedto optimize the upper and lower magnetic layers for the use intended.For example, double coating for VHS tapes has been carried out by usingferromagnetic powders different in particle size or magneticcharacteristics for the upper and lower layers to realize high outputand low noise in the whole region of luminance, color, and sound.

For the production of Hi 8 double-coated MP tapes, a so-called hybriddouble coating system using different kinds of magnetic substances inthe upper and lower magnetic layers has been developed, in whichmetallic magnetic substance meeting the demand of high-density recordingis used in the upper magnetic layer, and iron oxide magnetic substanceexcellent in low region characteristics is used in the lower magneticlayer to thereby obtain high fidelity of sharp image and clear colors onreproduction.

Nevertheless, the conventional techniques or concepts have limits forthe pursuit of further increased recording density and for drasticimprovements in high region characteristics with Hi 8 MP tapes. Hence,the inventors have prosecuted further analyses and studies in theprinciples and mechanism of magnetic recording itself for the purpose ofrealizing a magnetic recording medium surpassing deposited tapes in highregion characteristics.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a magnetic recordingmedium capable of high-density recording, which, while being of coatedtype, exhibits high region output comparable to deposited tapes as wellas running durability and preservability.

Another object of the present invention is to provide a thin magneticrecording medium which exhibits excellent electromagneticcharacteristics such as an output and a C/N, which makes good contactwith a head, which has satisfactory preservation stability, and whichcan be produced in good yield and with satisfactory efficiency.

A still another object of the present invention is to provide a magneticrecording medium which exhibits excellent electromagneticcharacteristics, particularly a high output in short wave recording, andrunning durability, and which can be produced in good yield.

A yet another object of the present invention is to provide a magneticrecording medium which has a high RF output, excellent runningdurability, a reduced dropout rate, and a low block error rate (BER).

A further object of the present invention is to provide a magneticrecording medium which exhibits satisfactory electromagneticcharacteristics and running properties, and particularly to provide amagnetic recording medium having satisfactory surface roughness and highelectromagnetic characteristics which is prepared by a simultaneouswet-on-wet coating system.

A still further object of the present invention is to provide a magneticrecording medium having satisfactory electromagnetic characteristics, alow percent thermal shrinkage, and excellent long-term preservability.

A yet further object of the present invention is to provide a magneticrecording medium having excellent electromagnetic characteristics, andparticularly reduced susceptibility to edge damages on repeated running.

The above objects of the present invention are accomplished by amagnetic recording medium comprising a non-magnetic support havingprovided thereon at least one lower non-magnetic layer comprising abinder having dispersed therein a non-magnetic powder and at least oneupper magnetic layer comprising a binder having dispersed therein aferromagnetic powder which is coated while said lower non-magnetic layeris wet, wherein said upper magnetic layer has an average dry thickness(d) of not more than 1.0 μm, and an average thickness variation (.sup.Δd) at the interface between said upper magnetic layer and lowernon-magnetic layer is not more than d/2 (.sup.Δ d≦d/2).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration for explaining the method for determining.sup.Δ d of a magnetic recording medium.

FIG. 2 is a schematic illustration of successive wet-on-wet coatingsystem according to the present invention.

FIG. 3 is a schematic illustration of simultaneous wet-on-wet coatingsystem according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

As a result of extensive investigations, the inventors have reachedconclusions that: with the conventional wet-on-wet coating system beingbasically adopted, one important point is to minimize "signal loss"which becomes greater with a reduction in recording wavelength, and areduction in magnetic layer thickness is required for minimizing thesignal loss at issue; and another important point is to increasemagnetic energy of a magnetic layer as high as possible, and developmentof a new magnetic substance and achievement of high density packing arerequired for obtaining increased magnetic energy.

The inventors first set about thorough reduction of signal loss. Ofvarious types of losses occurring during recording and reproduction in amagnetic recording system, the inventors found that improvements in highregion characteristics can be brought about by reducing"self-demagnetization loss" which has ever been considered unavoidablewith MP (metal) tapes.

That is, a first feature of the present invention consists inrealization of a uniform and very thin magnetic layer having a thicknessof not more than 1 μm, an average thickness variation of not greaterthan 1/2 the thickness, and a standard deviation of the thickness valuesof not more than 0.2 μm and great reduction in self-demagnetization lossin the short wavelength region. This can be achieved by makingthixotropy of coating compositions for a lower non-magnetic layer(hereinafter referred to as layer (a)) and an upper magnetic layer(hereinafter abbreviated as layer (b)) equal or close or by controllingthe shape of a non-magnetic powder in layer (a) so as to eliminate anymixed region in the interface between layers (a) and (b).

A second feature of the present invention, which is to be added to theabove-mentioned first feature, is that the interface between layers (a)and (b) is made uniform with small variations and the surface of layer(b) is made very smooth by controlling the size and shape of theferromagnetic powder and non-magnetic powder in layers (b) and (a),respectively, and improving dispersibility of the non-magnetic powderitself. Such a smooth surface of layer (b) eliminates "space loss" toimprove high region output.

A third feature of the present invention consists in accomplishment ofincreased energy of a magnetic layer. It was ascertained that themagnetic energy and coercive force of layer (b) can be increased byusing a ferromagnetic fine powder having an increased Hr and anincreased Hc in layer (b) thereby exhibiting high region output equal oreven higher than that obtained by ME (deposited) tapes.

A fourth feature of the present invention lies in high density packing.Mere thickness reduction of a magnetic layer as conventionally attemptedresults in reduction of low region output or deterioration of colorcharacteristics. In the present invention, an inorganic fine powderhaving extremely high rigidity in the thickness direction greatlyincreases the packing effect of calendering to achieve high densitypacking of a high energy ferromagnetic powder to thereby exhibitexcellent middle to low region characteristics.

A fifth feature of the present invention resides in achievement of suchcharacteristics as viscoelasticity, adhesion strength, resistance tosteel ball wear, residual solvent content, a sol fraction, etc. forassuring excellent durability that is never reached by ME (deposited)tapes.

The above-mentioned five features function with each other organically,supplementarily, synergistically, and systematically to establish a newlayer structure having an extremely reduced thickness, an extremelysmoothed surface, and an extremely increased packing and to realizeexcellent characteristics in the high region as well as the middle tolow region which have never been obtained by conventional coatingtechniques.

The first feature of the present invention is described below in detail.

The first feature can be realized by a magnetic recording mediumcomprising a non-magnetic support having provided thereon at least onelower non-magnetic layer (layer (a)) comprising a binder havingdispersed therein a non-magnetic powder and at least one upper magneticlayer (layer (b)) comprising a binder having dispersed therein aferromagnetic powder which is coated on layer (a) while layer (a) iswet, wherein layer (b) has an average dry thickness (d) of not more than1.0 μm, and an average thickness variation (.sup.Δ d) at the interfacebetween layers (a) and (b) in not more than d/2 (.sup.Δ d≦d/2).

That is, the first feature is an extremely reduced thickness of amagnetic layer. As a principle of self-demagnetization, the loss becomessmaller as the cross-sectional area of a magnetic layer reduces.Accordingly, achievement of an increased output of short wavelengthsignals indispensably requires drastic reduction of magnetic layerthickness. The effect of thickness reduction is insubstantial with athickness greater than 1 μm, and becomes appreciable according as thethickness approaches to an effective recording thickness, generallyrecognized to be 1/4 the recording wavelength, i.e., to recordingsaturation. Thus, it is demanded to reduce the thickness to an order ofa submicron unit.

However, the conventional single layer coating technique encountersdifficulty in coating to a thickness of submicron order. Further, as thethickness decreases, it becomes more difficult to obtain uniformthickness and surface smoothness. Besides, it has been very difficult tosupply a thin coating film in a large quantity in a stable manner.

In the present invention, the conventional double coating technique isadopted with such an innovation as making it possible to form layer (b)by wet-on-wet coating on layer (a) containing an inorganic fine powder,said layer (b) having an average thickness variation of not greater than1/2 the thickness and a standard deviation of thickness of not more than0.2 μm. The thickness of the resulting magnetic layer is from 1/3 to1/10 that of conventional Hi 8 MP tapes and, therefore, reducesself-demagnetization loss to achieve a great increase in luminancesignal output.

The principles of self-demagnetization are as follows. Magnetic poles ofa magnetized magnet makes a magnetic field in not only the outside ofthe magnet but the inside thereof. The magnetic field in the inside ofthe magnet has an opposite direction to the magnetization direction andacts to decrease magnetization. Such an inside magnetic field is calleda demagnetization field, and the decrease in magnetization caused by thedemagnetization field is called "self-demagnetization".

The intensity of demagnetization field depends on the shape of a magnet.The smaller the cross-section area, i.e. , the greater the between-polesdistance, the smaller the demagnetization field, i.e., the smaller theself-demagnetization. On comparing between, for instance, a sewingneedle and a steel ball, both of which are attracted to a magnet, aneedle easily becomes a magnet because of its smallself-demagnetization, whereas a steel ball has a largeself-demagnetization and hardly becomes a magnet.

In the case of magnetic tapes, the demagnetization field is small inlong wavelength (low region) recording, but as the recording wavelengthbecomes shorter, the distance between magnetic poles becomes smaller,and the demagnetization field increases to cause an increase ofself-demagnetization loss. This is one of the great causes ofdeterioration of high region characteristics.

In order to lessen self-demagnetization loss, it is effective to reducethe cross-section area, i.e., the thickness of the magnetic layeraccording to the principles of self-demagnetization. As saturationrecording is approached, the self-demagnetization loss becomes smallerto increase the output. Accordingly, it is required to reduce thethickness to an order of submicron, i.e., to bring the thickness closeto the effective magnetic layer thickness (1/4 the recordingwavelength).

The shortest recording wavelength of Hi 8 tapes is as extremely short as0.49 μm, which is one of the reasons for the excellent high regioncharacteristics of the same levels as attained by extremely thin ME(deposited) tapes having an about 0.2 μm thick magnetic layer.

On the other hand, coated type MP tapes have an about 3 μm thickmagnetic layer. The conventional coating system unavoidably results information of a magnetic layer having a thickness considerably greaterthan the recording wavelength. The deterioration of high regioncharacteristics due to self-demagnetization loss has been an insuperablewall one comes up against in seeking for improvements in image quality.The inventors have succeeded to overcome the wall.

Besides self-demagnetization loss, space loss is another great cause ofdeterioration of high region characteristics. Magnetic flux on the tapesurface is weakened as the recording wavelength becomes shorter so thateven very slight spacing between tape and a video head leads to a greatloss. Space loss includes micro-space loss attributable to surfaceroughness of a magnetic layer and macro-space loss attributable torigidity of the tape. In order to cope with the former space loss, it isimportant to assure stable running properties while keeping high surfacesmoothness. The importance is particularly high in high-densityrecording on, for example, Hi 8 tapes whose shortest recordingwavelength is about 40% of that of VHS tapes. With respect to the latterspace loss, generally called "contact with head", how to consistentlysatisfy both strength and flexibility is a subject awaiting forsolution. The space loss arising from tape rigidity has great influenceson image quality irrespective of the recording wavelength. The inventorshave settled down these problems of space loss all at once by the secondfeature described below in detail.

In the present invention, layer (b) preferably has a squared averagesurface roughness R_(rms), as measured with a scanning tunnel microscope(STM), satisfying a relationship: 30≦d/R_(rms), wherein d is a drythickness of layer (b) which is specified to be not more than 1.0 μm inthe present invention.

The second feature is smoothening of the surface of a magnetic layer.Double coating technique is essentially competent for obtainingexcellent surface smoothness because a lower layer absorbs surfaceunevenness of the base film so as to diminish the adverse influence ofthe unevenness upon an upper layer.

However, when one aims at surface smoothing with his considerations alsogiven to very slight space loss in recording at short wavelength of 0.5μm or less, the conventional double coating techniques have their owntechnical limits because recording mechanism needs use of ultrafinemagnetic powders excellent in high region characteristics in an uppermagnetic layer whereas a lower layer contains a relatively largenon-magnetic powder which may cause interfacial disturbances betweenlayers (a) and (b). Namely, even a very slight interfacial disturbancecaused by the relatively large non-magnetic powder must be thoroughlyavoided. The smaller the thickness of the upper layer, the greater theinfluence of the interfacial condition on smoothness of the uppermagnetic layer. Resolution of this problem has thus been of importance.

The inventors pursued size reduction of non-magnetic particles used inthe lower layer and high-density packing of the non-magnetic particles.However, ultrafine particles, as they are, are difficult to be packeduniformly and at high density. Hence, the individual ultrafine particlesare subjected to a special surface treatment to have improveddispersibility to thereby achieve high-density packing and interfacialflatness.

Further, having a high packing, the non-magnetic layer has a high degreeof freedom in the in-plane direction of tape to show excellentflexibility while exhibiting extremely high rigidity in the thicknessdirection to increase the calendering effect. As a result, smoothnesscan be increased by 20% of that of Hi 8 MP-DC, and the surface roughnessof the magnetic layer can be decreased to 2.5 nm. The high smoothnesswhich cannot be achieved but by wet-on-wet coating leads to a greatreduction of space loss in the short wavelength region and toimprovements in high region characteristics.

The third feature of the present invention is explained below in detail.The third feature lies in high output and low noise of a magnetic layerwhich are fundamental factors of magnetic tape performance. Suchcharacteristics can be achieved by an upper magnetic layer having aresidual coercive force (Hr) of 1500 Oe or higher in the normaldirection of the surface thereof. In order to improve characteristics atshort wavelengths, it is inevitable to formulate a magnetic layer so asto have high output and low noise by means of "size reduction ofmagnetic particles", "increase in energy", and "increase in packing" aswell as "minimization of signal loss".

The fourth feature of the present invention is described below.

The magnetic recording medium of the present invention preferably has aratio of stiffness in the coating direction (machine direction)(hereinafter abbreviated as SMD) to stiffness in the width direction(transverse direction) (hereinafter abbreviated as STD), i.e., SMD/STD,of from 1.0 to 1.9. more specifically, it is preferable that theinorganic powder present in layer (a) is a spherical to cubic polyhedralpowder having a Mohs hardness of 6 or higher and an average particlesize of not more than 0.15 μm.

Further, the magnetic recording medium preferably has a percent thermalshrinkage at 80° C.×30 mins. of not more than 0.4%. More specifically,it is preferable that layer (a) has a dry thickness 1 to 30 times thatof layer (b) and that a difference between the powder volume ratio oflayer (a) and that of layer (b) is in the range of from -5% to +20%.

In other words, the fourth feature resides in high-density packing, andit is the lower non-magnetic layer that makes it feasible to fill themagnetic powder at a high packing. The non-magnetic layer having asmooth surface and very high rigidity in the thickness direction surelyreceives the strong pressure of a super-high density packing (HDP)calender to thereby realize extraordinary high-density packing.

In pursuit of high region characteristics by great thickness directionof a magnetic layer, the conventional techniques have been confrontedwith reductions in middle to low region characteristics, resulting in afailure to obtain excellent color characteristics. In the presentinvention, this problem is settled by high-density packing of a highenergy magnetic substance, and the lower non-magnetic layer makes itpossible to arrive at such a settlement. As a result, a greatimprovement in high region output can be obtained while retainingexcellent characteristics in the middle to low region.

That is, the present invention achieves high-density recordingcomparable to deposited tapes, that has been believed impossible toobtain with the conventional coated type magnetic recording media. Suchachievement can first be reached by (i) uniformly coating layer (b) to adry thickness of 1.0 μm while layer (a) is wet (wet-on-wet coatingsystem) and (ii) controlling an average thickness variation .sup.Δ d atthe interface between layers (a) and (b) at or under d/2 with d beingreduced to 1.0 μm or less in average. There is thus obtained ahigh-density recording medium which is comparable to deposited tapes,notwithstanding it is of coated type, and which can be actually put topractical use. One can find in patent applications filed to date someproposals relating to magnetic recording media having a 1.0 μm or lessthick upper magnetic layer and a lower non-magnetic layer, but noembodiment which may be brought into the market has ever been developed.Hence, the present invention is an epoch-making achievement overthrowingthe commonly received knowledge.

Thickness of layer (b) and interfacial variation .sup.Δ d according tothe present invention are determined as follows. A magnetic recordingmedium is sliced along its longitudinal direction with a diamond cutterto a thickness of about 0.1 μm. Micrographms (print size: A4 to A5) ofthe cut area were taken under a transmission electron microscope at amagnification of 10000 to 100000, preferably 20000 to 50000. Theinterface between layers (a) and (b) on the micrograph was identifiedwith the naked eye with attention being paid to a difference in shapebetween the magnetic powder in layer (b) and the non-magnetic powder inlayer (a) and traced with a black marker pen. The surface of layer (b)is also traced with a black marker pen. By the use of an image analyzer"IBAS 2" manufactured by Zeiss Co., a span of 21 cm in the longitudinaldirection is divided into 100 to 300 segments, and the distance in thethickness direction between the two black lines (the interface and thesurface of layer (b)) is measured in each segment to obtain an averagethickness (d) of layer (b).

A distance (.sup.Δ d_(i)) in the thickness direction between a peak anda valley of the line formed by the interface is measured. Themeasurement was made on every peak and valley appearing within a widthof 20 μm (real length) (10 to 20 measurements) to obtain an averagethickness variation .sup.Δ d at the interface.

While it is ideal that the interface forms a straight line with aconstant thickness of layer (b), a practically achievable preferredlevel is that the interface forms a gently-sloping curve akin to a sinecurve having a smaller amplitude and a longer interval between peaks andvalleys as compared with the conventionally formed interface. The numberof peaks or valleys appearing within 20 μm is preferably limited to 10to 20 at the most. With respect to the measurement of interfacialthickness variation, FIG. 1 can be referred to.

Accordingly, .sup.Δ d can be calculated from:

    .sup.Δ d=(.sup.66 d.sub.1 +.sup.Δ d.sub.2 + . . . + .sup.Δ d.sub.m)/m (m=10 to 20)

The distance (L) between two adjacent peaks of the curve of theinterface is preferably not less than 1 μm, and particularly not lessthan 2 μm.

A standard deviation C of the thickness of layer (b) can be obtained byutilizing the same measured values as obtained above for 100 to 300divided segments. A preferred standard deviation a is not more than 0.2μm.

Returning to the first feature of the present invention, the magneticrecording medium of the present invention comprises a non-magneticsupport having provided thereon at least one layer (a) and at least onelayer (b) which is coated while said layer (a) is wet, wherein layer (b)has an average dry thickness (d) of not more than 1.0 μm, and an averagethickness variation (.sup.Δ d) at the interface between layers (a) and(b) is not more than d/2 (⁶⁶ d≦d/2). Layer (b) preferably has a standarddeviation a of dry thickness of not more than 0.2 μm.

The first feature can be accomplished by the following two embodiments.

A first embodiment is to control coating compositions (dispersions) forboth layers (a) and (b) so as to have equal or approximate thixotropy,and a second embodiment is to control the size and shape of thenon-magnetic powder in layer (a) and the magnetic powder in layer (b) soas to dynamically prevent formation of a mixed region at the interfacebetween layers (a) and (b).

In the above-mentioned first embodiment, it is preferable that thedispersion for layer (a) shows such thixotropy that a ratio of shearstress A10⁴ at a shear rate of 10⁴ sec⁻¹ to shear stress A10 at a shearrate of 10 sec⁻¹, A10⁴ /A10, ranges from 3 to 100 and more preferablyfrom 4 to 90. With the thixotropy of the dispersion for layer (a) beingso controlled as having a specific A10⁴ /A10 ratio, the dispersion wouldexhibit thixotropy close to that of the magnetic coating composition forlayer (b), causing no interfacial disturbance, and thereby avoidingpinholes or run-away.

Thus, the outstanding feature of the first embodiment of the presentinvention consists in the use of coating compositions for layers (a) and(b) each having specifically controlled rheological characteristics soas to show the same or substantially the same thixotropy, wherebycoating defects are eliminated, a yield of production is improved, anddurability and electromagnetic characteristics, e.g., output, areimproved.

According to the above-described preferred embodiment, the same range ofA10⁴ /A10 ratio also applies to the magnetic coating composition forlayer (b).

The terminology "substantially the same thixotropy" as used herein meansthat difference in the A10⁴ /A10 ratio between the dispersion for layer(a) and the magnetic coating composition for layer (b) is not more than97 and preferably 80 or less.

In the present invention, the shear stress at a certain shear rate ismeasured with a coaxial cylinder viscometer, e.g., "Rotovisco viscometerRV-II" manufactured by HAAKE CO. The shear rate is decided by diametersof the inner and outer cylinders, the clearance therebetween, and thenumber of rotation. A shear rate (D), an apparent viscosity (η), and ashear stress (A) have the following relationships.

    A=η·D

    D=dv/dr (v: peripheral speed; r: radius)

In this regard, reference can be made to T. C. Patton, Paint Flow andPigment Dispersion, pub. by John Wiley & Sons (1964).

Accordingly, A10 and A10⁴ correspond to A=10η and A=10⁴ η, respectively.The A10⁴ /A10 ratio is indicative of the degree of thixotropy. Thegreater the ratio, the smaller the thixotropy, and vise versa.Conventional non-magnetic dispersions have a A10⁴ /A10 ratio of fromabout 200 to 500, i.e., weak thixotropy. If the A10⁴ /A10 ratio becomesgreater, the dispersion approximates closely a Newtonian fluid, makingit difficult to simultaneously coat an upper magnetic layer thereon. Ifit is too small, thixotropy of the dispersion is too strong to establishconsistency between the viscosity suitable for simultaneous coating andthe viscosity suitable for liquid feed.

The following four means (A) to (D) are included in the magneticrecording medium according to the first embodiment of the presentinvention which satisfies the above-mentioned thixotropic conditions,that is, the thixotropy of the dispersion for layer (a) is made at leastsubstantially equal to that of the magnetic coating composition forlayer (b), and, preferably, the A10⁴ /A10 ratio falls within theabove-recited specific range for layers (a) and (b). These means are tobe construed as illustrative non-limiting examples of the presentinvention.

(A) The non-magnetic powder in layer (a) contains (i) carbon black and(ii) an inorganic powder (other than carbon black) having a smalleraverage primary particle diameter than the dry thickness of layer (a),and layers (a) and (b) each contain a thermosetting polyisocyanate in aproportion of from 10 to 70% by weight based on the total binder presentin each layer.

(B) The non-magnetic powder in layer (a) contains an inorganic powderhaving an average primary particle diameter of not more than 0.08 μm.

(C) Layer (a) has a maximum magnetic flux density (Bm) of from 30 to 500gauss. In this case, while layer (a) has low magnetic properties, ittakes no part in magnetic recording and is therefore regarded"non-magnetic".

(D) The ferromagnetic powder in layer (b) has an average major axis ofnot more than 0.3 μm and an average crystallite size of not more than300 Å, layer (a) contains, as a non-magnetic powder (i) a non-magneticmetal oxide powder and (ii) carbon black having an average primaryparticle diameter of less than 20 nm at a (i)/(ii) ratio of from 95/5 to60/40 by weight, and layer (a) contains a polyurethane having at leastthree hydroxyl groups in the molecule thereof and a polyisocyanatecompound.

The terminology "average primary particle diameter" as used herein meansan average particle diameter obtained from a size distribution of singleparticles free from fusion or association.

Means (A) is characterized by using a dispersion for layer (a) which isprepared from carbon black making a great contribution to thixotropy andan inorganic powder making a slight contribution to thixotropy asnon-magnetic powder and a polyisocyanate as a binder. By these features,reduction in thickness of layer (b) and improvement in yield can beachieved. Further, by using a polyisocyanate in both layers (a) and (b),the magnetic recording medium maintains a satisfactory contact with ahead, is assured of preservability in a high temperature and highhumidity condition, and exhibits moderate rigidity and flexibility toimprove running durability.

In means (B), the dispersion for layer (a) is prepared by using aninorganic powder having a reduced size so as to have controlledthixotropy. A proper combination of rheological characteristics of layer(b) and those of layer (a) is of importance for achieving simultaneousor successive coating in a wet state. That is, a magnetic coatingcomposition exhibits very strong thixotropy because it has a structuralviscosity due to magnetic attraction among ferromagnetic particles.Having no magnetic attraction, on the other hand, a conventionallyemployed dispersion for a layer comparable to layer (a) has weakthixotropy. If a conventional non-magnetic layer dispersion and amagnetic coating composition are simultaneously coated, they undergomixing at the interface therebetween, or considerable cohesive streaksdevelop during the orientation following coating, causing a failure toobtain satisfactory surface properties. This problem is avoided in thepresent invention by using an inorganic powder having an average primaryparticle diameter of not more than 0.08 μm. If desired, the inorganicpowder can be used in combination with a minor proportion of carbonblack.

In means (C), the maximum magnetic flux density Bm of layer (a) iscontrolled between 30 gauss and 500 gauss. In other words, thedispersion for layer (a) has its thixotropy controlled so as to satisfythe above-described A10⁴ /A10 ratio range of between 3 to 100 by using amagnetic powder which is incapable of magnetic recording and gives noadverse influence on layer (b). The Bm of layer (a) can be controlledwithin the above-specified range by using a magnetic powder having a lowsaturation magnetization (σ_(s)) or by reducing the packing ratio of themagnetic powder in layer (a). As previously described, a layercontaining a powder showing magnetic properties but having such a lowσ_(s) incapable of magnetic recording is defined to be non-magnetic.

While means (A), (B) and (C) illustrate suitable means for controllingthe A10⁴ /A10 ratio within 3 and 100, the A10⁴ /A10 ratio is alsorelated to other factors described below. Therefore, these other factorsshould be taken into consideration in preparation of the dispersions forlayers (a) and (b) having the preferred A10⁴ /A10 ratio and inproduction of a magnetic recording medium having desiredcharacteristics. Factors relating to inorganic powders or magneticpowders to be dispersed include (i) particle size (specific surfacearea, average primary particle diameter, etc.), (ii) structure (oilabsorption, particle shape, water content, etc.), (iii) properties ofpowder surface (pH, weight loss on heating, etc.), and (iv) attractionforce of particles (σ_(s), etc.). Factors relating to binders include(v) molecular weight and (vi) functional groups. Factors relating tosolvents include (vii) type (polarity, etc.), (viii) capability ofdissolving a binder, and (ix) amount.

In some detail, a A10⁴ /A10 ratio is influenced by various parameters,such as molecular weight of binders, structure, affinity for solvent,water content, and the like. Thus, an important factor is the powdermaterial selected to be packed. That is, a A10⁴ /A10 ratio isconsiderably dependent on the surface properties, particle size, oilabsorption, and affinity for a solvent properties of the dispersedpowder. For example, the smaller the particle size, the stronger thethixotropy. Carbon black which has a high oil absorption and is texturedin structure makes the thixotropy very strong. Further, magneticparticles exhibit strong structural viscosity due to their own magneticattraction and generally afford strong thixotropy. In the presentinvention, these powder properties are utilized in controllingthixotropy of dispersions for layers (a) and (b) within theabove-mentioned A10⁴ /A10 range, for example.

The above-described second embodiment of the present invention can beaccomplished by the following means (E) to (G). The essence of thesemeans consists in how to eliminate a mixed region between layers (a) and(b) , and means (E) to (G) are to be construed as illustrativenon-limiting examples of the present invention.

(E) The non-magnetic powder in layer (a) has a longest axis r₁ toshortest axis r₂ ratio (r₁ /r₂) of not less than 2.5.

(F) The non-magnetic powder has an acicular ratio of not less than 2.5,and the magnetic powder has an average major axis of not more than 0.3μm.

(G) Layer (a) contains a flaky non-magnetic powder and a bindercontaining an epoxy group and a molecular weight of more than 30,000,and layer (b) contains an acicular ferromagnetic powder or a tabularferromagnetic powder.

In these means, an acicular or flaky non-magnetic powder is used inlayer (a) in order to prevent formation of a mixed region in theinterface between layers (a) and (b). As compared with conventionalparticulate non-magnetic powder, acicular non-magnetic powders areneatly arranged to form a strong coating film even while undried tothereby prevent formation of a mixed region even if the ferromagneticpowder in layer (b) rotates. Flaky non-magnetic powders are spread tocover the entire area in a manner akin to flooring tiles and produce thesame effect as acicular powders in preventing interfacial mixing even ifthe ferromagnetic powder in layer (b) rotates.

In order that the flaky particles be so arranged like tiles,dispersibility of the non-magnetic dispersion should be improved byusing an epoxy-containing binder having a molecular weight of 30,000 ormore.

Thus, a very thin and smooth magnetic layer can be obtained withoutdeveloping a mixed region in the interface by using a non-magneticpowder having a characteristic shape in a lower non-magnetic layer andforming an upper magnetic layer thereon.

Means (A) is described below in more detail.

Carbon black which can be used in layer (a) has an average primaryparticle diameter of not more than 30 mμ, a specific surface area offrom 150 to 400 m² /g, and preferably from 180 to 350 m² /g, a DBPabsorption of from 40 to 300 ml/100 g, and preferably from 45 to 200ml/100 g, a particle diameter of not more than 30 mμ, preferably from 5to 27 mμ, and more preferably from 10 to 22 mμ, a pH of from 2 to 10, awater content of from 0.1 to 10% by weight, a tapped density of from 0.1to 1 g/cc. It is desirable that carbon black is first dispersed in acompatible binder and then mixed with an inorganic powder.

The inorganic powder preferably has a particle size of from 0.01 to 1μm, more preferably from 0.02 to 0.5 μm, and most preferably from 0.02to 0.08 μm. If desired, inorganic powders having different particlesizes may be combined, or a single inorganic powder having a broad sizedistribution may be employed to the same effect. Further, the inorganicpowder has a tapped density of from 0.05 to 2 g/cc, and preferably from0.2 to 1.5 g/cc, a water content of from 0.1 to 5% by weight, andpreferably from 0.2 to 3% by weight, a pH of from 2 to 11, a specificsurface area of from 1 to 100 m² /g, preferably from 5 to 50 m² /g, andmore preferably from 7 to 40 m² /g, a crystallite size of from 0.01 to2μ, a DBP absorption of from 5 to 100 ml/100 g, preferably from 10 to 80ml/100 g, and more preferably from 20 to 60 ml/100 g, and a specificgravity of from 1 to 12, and preferably from 2 to 8. The shape of theinorganic powder may be any of an acicular shape, a spherical shape, anda cubic shape. The inorganic powder does not need to be 100% by weightpure and may be surface-treated with other compounds, if desired. Inthis case, a purity of 70% by weight would be enough. In using titaniumoxide powder, for example, surface treatment with alumina is usuallyconducted. An ignition loss of the inorganic powder is preferably notmore than 20% by weight. The inorganic powder to be used preferably hasa Mohs hardness of at least 4. Examples of suitable inorganic powdersfor means (A) include titanium oxide, zinc oxide, tin oxide, andaluminum oxide. If desired, the non-magnetic powder comprising carbonblack and an inorganic powder may be used in combination with otherarbitrary non-magnetic powders.

A preferred weight ratio of carbon black to inorganic powder ranges from10:90 to 80:20, and more preferably from 15:85 to 60:40.

The binder in layer (a) is preferably used in an amount of from 10 to100% by weight, and particularly from 13 to 50% by weight, based on thenon-magnetic powder. It is preferable to use a combination of 5 to 25%by weight of a vinyl chloride resin, 1 to 25% by weight of apolyurethane resin, and 1 to 15% by weight of a polyisocyanate, eachbased on the non-magnetic powder.

The polyisocyanate content in the total binder of each of layers (a) and(b) preferably ranges from 10 to 70% by weight, and more preferably from20 to 50% by weight.

Means (B) is illustrated below in more detail.

In means (B), both of the magnetic coating composition and thenon-magnetic dispersion preferably contain a polyisocyanate.

Suitable inorganic powders for means (B) include SiO₂, TiO₂, α-alumina,α-Fe₂ O₃, boron nitride, and tin oxide. The inorganic powder should havean average primary particle diameter of not more than 0.08 μm. Ifdesired, inorganic powders whose particle size is above this limit maybe used in combination, or a single inorganic powder having a broad sizedistribution may be employed to the same effect. The inorganic powderhas a tapped density of from 0.05 to 2 g/cc, and preferably from 0.2 to1.5 g/cc, a water content of from 0.1 to 5% by weight, and preferablyfrom 0.2 to 3% by weight, a pH of from 2 to 11, a specific surface areaof from 1 to 100 m² /g, preferably from 5 to 50 m² /g, and morepreferably from 7 to 40 m² /g, a crystallite size of from 0.01 to 2 Å, aDBP absorption of from 5 to 100 ml/100 g, preferably from 10 to 80ml/100 g, and more preferably from 20 to 60 ml/100 g, and a specificgravity of from 2 to 8. The shape of the inorganic powder may be any ofan acicular shape, a spherical shape, and a cubic shape. The inorganicpowder does not need to be 100% by weight pure and may besurface-treated with other compounds. In this case, a purity of 70% byweight would be enough. In using titanium oxide powder, for example,surface treatment with alumina is generally conducted. An ignition lossof the inorganic powder is preferably not more than 20% by weight. Theinorganic powder to be used preferably has a Mohs hardness of at least4.

Carbon black having an average primary particle diameter of not morethan 0.03 μm, and preferably not more than 0.023 μm, may be used incombination with an inorganic powder at an inorganic powder to carbonblack ratio of from 99:1 to 70:30, and particularly from 95:5 to 80:20,by weight. If the carbon black content is less than 1% by weight, whichis virtually none, a desired degree of thixotropy may not be obtained.If carbon black content exceeds 30% by weight, the resulting magneticrecording medium may have insufficient surface properties in some cases.

Carbon black which may be used in combination with the above-describedinorganic powder has a specific surface area of from 100 to 500 m² /g,and preferably from 150 to 400 m² /g, a DBP absorption of from 20 to 400ml/100 g, and preferably from 30 to 200 ml/100 g, a particle diameter offrom 5 to 80 mμ, preferably from 10 to 50 mμ, and more preferably from10 to 40 mμ, a pH of from 2 to 10, a water content of from 0.1 to 10% byweight, and a tapped density of from 0.1 to 1 g/cc.

These non-magnetic powders are used at a weight ratio to binder of from20 to 0.1, preferably from 10 to 0.5, and more preferably from 8 to 1,and at a volume ratio to binder of from 10 to 0.2.

Means (C) is illustrated below in more detail.

The Bm of layer (a) can be controlled within the range of from 30 to 500gauss by using a magnetic substance having a low saturationmagnetization (σ_(s)) or by reducing the packing ratio of the magneticsubstance in layer (a). In this case, a non-magnetic substance ispreferably used in an amount of from 100 to 5000 parts by weight per 100parts by weight of the magnetic substance. The non-magnetic substance tobe used preferably contains, in addition to binders, a non-magneticpowder, such as inorganic powders and carbon black. The magneticsubstance preferably includes a Co-γ-Fe₂ O₃ magnetic substance. Thecoercive force of the magnetic substance is not particularly limited butpreferably ranges from about 400 to 1500 Oe. Hexagonal barium ferritemay be used in layer (a). The coercive force Hc of hexagonal bariumferrite to be used in this case is also non-limited but preferablyranges from 800 to 4500 Oe.

Layer (b) preferably has a coercive force Hc of from 1200 to 3000 Oe anda maximum magnetic flux density Bm of from 200 to 4500 gauss.

The inorganic powder to be used in means (C) preferably has a particlediameter of from 0.01 to 2μ. If desired, inorganic powders of differentsizes may be combined, or a single powder having a broad sizedistribution can be used to the same effect. The inorganic powder has atapped density of from 0.05 to 2 g/cc, and preferably from 0.2 to 1.5g/cc, a water content of from 0.1 to 5% by weight, and preferably from0.2 to 3% by weight, a pH of from 2 to 11, a specific surface area offrom 1 to 100 m² /g, preferably from 5 to 50 m² /g, and more preferablyfrom 7 to 40 m² /g, a crystallite size of from 0.01 to 2 g, a DBPabsorption of from 5 to 100 ml/100 g, preferably from 10 to 80 ml/100 g,and more preferably from 20 to 60 ml/100 g, and a specific gravity offrom 1 to 12, and preferably from 2 to 8.

The shape of the inorganic powder may be any of an acicular shape, aspherical shape, and a cubic shape. An acicular shape is preferred fromthe standpoint of magnetic characteristics. In particular, an acicularinorganic powder whose shape and size are close to those of aferromagnetic powder is preferred. In this sense, a ratio of the majoraxis of the acicular inorganic powder to that of the ferromagneticpowder preferably falls within a range of from 0.5 to 3. The inorganicpowder does not need to be 100% by weight pure and may besurface-treated with other compounds. In this case, a purity of 70% byweight would be enough. In using titanium oxide powder, for example,surface treatment with alumina is usually conducted. An ignition loss ofthe inorganic powder is preferably not more than 20% by weight. Theinorganic powder to be used here preferably has a Mohs hardness of atleast 4.

In using carbon black as a non-magnetic powder in means (C), it ispreferably used in an amount of from 0.1 to 30% by weight based on themagnetic substance in either of layer (a) and layer (b). Besidescontributing to thixotropy, carbon black functions in static chargeprevention, reduction in coefficient of friction, light screening, andimprovement in film strength. These performances of the carbon blackvary depending on the species selected. Accordingly, the kind, amount orcombination of carbon black species to be used in layers (a) and (b) canbe selected appropriately depending on the purpose desired while takinginto consideration the above-mentioned various characteristics, e.g.,particle size, oil absorption, conductivity, and pH. For example, carbonblack having high conductivity may be used in layer (a) for staticcharge prevention, and carbon black of large size may be used in layer(b) for reduction of coefficient of friction.

Carbon black which may be used in combination has a specific surfacearea of from 100 to 500 m² /g, and preferably from 150 to 400 m² /g, aDBP absorption of from 20 to 400 ml/100 g, and preferably from 30 to 200ml/100 g, a particle diameter of from 5 to 80 mμ, preferably from 10 to50 mμ, and more preferably from 10 to 40 mμ, a pH of from 2 to 10, awater content of from 0.1 to 10% by weight, and a tapped density of from0.1 to 1 g/cc.

Other non-magnetic powders which can be used in means (C) includeorganic powders, e.g., an acrylate-styrene resin powder, abenzoguanamine resin powder, a melamine resin powder, a phthalocyaninepigment, a polyolefin resin powder, a polyester resin powder, apolyamide resin powder, a polyimide resin powder, and apolyfluoroethylene resin powder. These organic powders can be preparedby, for example, the processes described in JP-A-62-18564,JP-A-60-255827, and JP-A-60-255827.

These non-magnetic powders are used at a weight ratio to binder of from0.1 to 10 and at a volume ratio to binder of from 0.2 to 10.

Means (D) is now illustrated below in more detail.

One of the features of means (D) consists in that the ferromagneticpowder in layer (b) has an average major axis of not more than 0.3 μmand an average crystallite size of not more than 300 Å.

To obtain a satisfactory output in short wave recording, it ispreferable to use as fine as possible ferromagnetic powders. Further, amagnetic dispersion containing such a fine powder is well matched with anon-magnetic dispersion in rheological characteristics. If theferromagnetic powder has an average major axis of more than 0.3 μm or anaverage crystallite size of more than 300 Å, mixing of the magneticdispersion and the non-magnetic dispersion may take place, failing toobtain a magnetic layer of 1.0 μm or less in thickness.

A second feature of means (D) resides in that the powder constitutinglayer (a) comprises (i) a non-magnetic metal oxide powder and (ii)carbon black having an average particle size of less than 20 nm at aweight ratio of (i)/(ii) of 95/5 to 60/40.

Carbon black particles having an average size of smaller than 20 nm forma loose coherent structure in a dispersion to show pseudo-plasticity. Amixture of a non-magnetic metal oxide powder and such carbon black atthe above-recited specific mixing ratio provides proper rheologicalcharacteristics, particularly at a low shear rate.

If the ratio is more than 95/5, pseudo-plasticity at a low shear ratemay not be assured, causing mixing of layers (a) and (b). If it is lessthan 60/40, the dispersion tends to exhibit excessive pseudo-plasticityand become difficult to feed. Further, the dispersion may havedeteriorated dispersibility to have poor surface properties.

These non-magnetic powders including the metal oxide powder and thecarbon black are preferably used in layer (a) in an amount of 5 to 150%by weight and more preferably from 10 to 100% by weight based on binderin layer (a).

Suitable non-magnetic metal oxide powders used in this embodimentinclude α-alumina (a ratio: 90% or more), β-alumina, γ-alumina, chromiumoxide, cerium oxide, α-ion oxide, corundum, titanium oxide, silicondioxide, tin oxide, magnesium oxide, tungsten oxide, zirconium oxide,and zinc oxide. The metal oxide powder preferably has a particlediameter of 0.01 to 2 μm. If desired non-magnetic metal oxide powderswhose particle size is above this limit may be used in combination, or asingle non-magnetic metals oxide powder having a broad size distributionmay be employed to the same effect. The metal oxide powder has a tappeddensity of from 0.05 to 2 g/cc, and preferably from 0.2 to 1.5 g/cc, awater content of from 0.1 to 5% by weight, and preferably from 0.2 to 3%by weight, a pH of from 2 to 11, a specific surface area of from 1 to100 m² /g, preferably from 5 to 50 m² /g, and more preferably from 7 to40 m² /g, a crystallite size of from 0.01 to 2μ, a DBP absorption offrom 5 to 100 ml/100 g, preferably from 10 to 80 ml/100 g, and morepreferably from 20 to 60 ml/100 g, and a specific gravity of from 2 to12, and preferably from 3 to 8.

The shape of the metal oxide powder may be any of an acicular shape, aspherical shape, a cubic shape, and a tabular shape. The metal oxidepowder does not need to be 100% by weight pure and may besurface-treated with other compounds. In this case, a purity of 70% byweight would be enough. In using titanium oxide powder, for example,surface treatment with alumina is generally conducted. An ignition lossof the metal oxide powder is preferably not more than 20% by weight. Themetal oxide powder to be used preferably has a Mohs hardness of at least4.

Typical examples of commercially available non-magnetic metal oxidepowder include UA5600 and UA5605, (all produced by Showa Denko K.K.);AKP-20, AKP-30, AKP-50, HIT-50, HIT-100, and ZA-G1 (all produced bySumitomo Chemical Co., Ltd.); G5, G7, and S-1 (all produced by NipponChemical Industrial Co., Ltd.); TF-100, TF-120, and TF-140 (all producedby Toda Kogyo K.K.); TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S,TTO-55D, FT-1000, FT-2000, FTL-100, FTL-200, M-1, S-1, and SN-100 (allproduced by Ishihara Sangyo Kaisha, Ltd.); ECT-52, STT-4D, STT-30D,STT-30, STT-65C, Y-LOP, and R516 (all produced by Titan Kogyo K.K.); T-1(produced by Mitsubishi Material Corp.); MT-100S, MT-100T, MT-150W,MT-500B, MT-600B, and MT-100F (all produced by TAYCA CORPORATION);FINEX-25 (produced by Sakai Chemical Industry Co., Ltd.); and DEFIC-Y(produced by DOWA MINING CO., LTD.).

A third feature of means (D) is that layer (a) contains a polyurethanehaving at least three hydroxyl groups in the molecule thereof and apolyisocyanate compound. In order to form a very thin upper magneticlayer by wet-on-wet coating, it is necessary to control the rheologicalrelationship between the upper magnetic dispersion and the lowernon-magnetic dispersion. Where both the upper and lower layers contain aferromagnetic powder, the range of rheological characteristics of thecoating compositions permissible in simultaneous wet-on-wet coating isrelatively broad. Where the dispersion for the lower layer contains onlya non-magnetic powder as in the present invention, it exhibits muchdifferent rheological characteristics from those of the magneticdispersion for the upper layer because of lack of magnetic properties.If two dispersions different in viscoelasticity are wet-on-wet coated,the two coatings undergo mixing, resulting in the failure of filmformation. It is not until the conditions mentioned above in the secondand third features are satisfied that the rheological characteristics ofthe lower non-magnetic dispersion can be controlled in good agreementwith those of the upper magnetic dispersion containing theabove-mentioned specific ferromagnetic powder.

It is known that a polyisocyanate compound is reacted with a hydroxylgroup to provide a high-molecular weight compound. When a polyisocyanatecompound and a polyurethane having three or more hydroxyl groups permolecule are dispersed together, the reaction slightly proceeds in thedispersion to provide moderate rheological characteristics. If thepolyurethane contains two or less hydroxyl groups, the rheologicalcharacteristics of the dispersion becomes different from those of theupper magnetic dispersion, resulting in mixing of layers.

That is, in means (D), layer (a) containing fine carbon black particleshaving a particle size of less than 20 nm, in which the carbon blackparticles are, though non-magnetic, linked in a bead-like structure toshow thixotropic properties is formed under layer (b) containing fineferromagnetic powder. The thixotropic properties of the carbon black aresynergistically enhanced by a three-dimensional high polymer formedbetween a polyurethane having at least three hydroxyl groups permolecule and a polyisocyanate compound in layer (a). Thus, a very thinmagnetic layer (b) can be formed on layer (a) without suffering from anycoating defect such as pinholes or coating streaks.

The, polyurethane which can be used in means (D) is basically preparedfrom a polyol, a diisocyanate and, if desired, a chain extender by knownprocesses. The polyol and/or chain extender may have a plurality ofhydroxyl groups having different reactivity. The polyurethane may alsobe prepared by opening the epoxy group of an epoxy-containingpolyurethane which is obtainable by using an epoxy-containing polyol.

Where a chain extender is used, the skeleton of the polyol to be used isnot limited as far as it has at least two hydroxyl groups and isselected from polyether polyol, polyester polyol, polycarbonate polyol,polycaprolactone polyol, and copolymer polyols thereof. Where no chainextender is used, the polyol to be used must have at least threehydroxyl groups per molecule. Typical examples of the polyether polyolskeleton include polyalkylene glycols, e.g., polyethylene glycol andpolypropylene glycol. The polyester polyol skeleton can be synthesizedby, for example, polycondensation of a dihydric alcohol and a dibasicacid or ring-opening polymerization of lactones, e.g., caprolactone.Examples of typical dihydric alcohols include glycols, e.g., ethyleneglycol, propylene glycol, butanediol, 1,6-hexanediol, andcyclohexanedimethanol. Typical examples of dibasic acids include adipicacid, pimelic acid, azelaic acid, sebacic acid, phthalic acid, andterephthalic acid.

The polycarbonate polyol skeleton includes (i) a polycarbonate polyolhaving a molecular weight of from 300 to 20,000 and a hydroxyl value offrom 20 to 300 which is synthesized by condensation orinteresterification between a dihydric alcohol represented by formula:

    HO--R.sup.1 --OH

wherein R¹ represents --(CH₂)_(n) --(n=3 to 14), ##STR1## and the likeand phosgene, a chloroformic ester, a dialkyl carbonate, or a diarylcarbonate and (ii) a polycarbonate polyester polyol having a molecularweight of from 400 to 30,000 and a hydroxyl value of from 5 to 300 whichis synthesized by condensation between the above-described polycarbonatepolyol (i) and a dicarboxylic acid represented by formula:

    HOOC--R.sup.2 --COOH

wherein R² represents an alkylene group having from 3 to 6 carbon atoms,a 1,4-, 1,3-, or 1,2-phenylene group, or a 1,4-, 1,3-, or1,2-cyclohexylene group.

The above-described polyols may be used in combination with up to 90% byweight of other polyols, e.g., polyether polyol, polyester polyol orpolyester, based on the above-described polyol. The above-describedpolyol may contain, in addition to a hydroxyl group, a polar functionalgroup, e.g., --SH, --COSH, --CSSH, --SO₃ M, --COOM, --OPO(OM)₂ (whereinM represents a hydrogen atom, Na, K, or Li), an epoxy group, and anamino group.

The diisocyanate which can be reacted with the polyol to form apolyurethane is not particularly limited, and any of the conventionallyemployed diisocyanate compounds can be used. Examples of suitablediisocyanate compounds are tolylene diisocyanate, 4,4'-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophoronediisocyanate, and triphenylmethane triisocyanate.

The chain extenders which can be used include known compounds, such asthe above-mentioned polyhydric alcohols, aliphatic polyamines, alicyclicpolyamines, and aromatic polyamines, and, in addition, these compoundshaving further bonded thereto a hydroxyl group which is different inreactivity.

The polyurethane resin is added to the non-magnetic metal oxide powderin an amount usually of from 1 to 100% by weight, preferably from 3 to50% by weight, and more preferably from 4 to 40% by weight, based on thenon-magnetic metal oxide powder. The polyurethane to be added preferablyhas a glass transition temperature of from -50° to 100° C., anelongation at break of from 100 to 2000%, a breaking stress of from 0.05to 10 kg/cm², and a yield point of from 0.05 to 10 kg/cm².

The polyisocyanate which is used in layer (a) includes isocyanatecompounds, e.g., tolylene diisocyanate, 4,4'-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophoronediisocyanate, and triphenylmethane triisocyanate; reaction productsbetween such an isocyanate compound and a polyhydric alcohol; andpolyisocyanate compounds obtained by condensation of isocyanatecompounds. These polyisocyanate compounds are commercially availableunder trade names of Coronate L, Coronate HL, Coronate 2030, Coronate2031, Millionate MR, and Millionate MTL (all produced by NipponPolyurethane Co., Ltd.); Takenate D-102, Takenate D-110N, TakenateD-200, and Takenate D-202 (all produced by Takeda Chemical Industries,Ltd.); and Desmodule L, Desmodule IL, Desmodule N, and Desmodule HL (allproduced by Sumitomo Bayer Co., Ltd.). These compounds may be usedeither individually or in combination of two or more thereof to takeadvantage of a difference in curing reactivity. The polyisocyanate isadded in an amount usually of from 1 to 50% by weight, preferably from 2to 40% by weight, and more preferably from 4 to 30% by weight, by weightbased on the non-magnetic metal oxide powder.

According to the second embodiment, a non-magnetic powder having acharacteristic shape is used in the lower non-magnetic layer therebymaking it possible to form an upper magnetic layer having a drythickness of not more than 1.0 μm without forming a mixed region betweenthe upper magnetic layer and the lower non-magnetic layer.

The terminology "mixed region" as used herein means a region where thecomponents of the upper magnetic layer and those of the lowernon-magnetic layer are present in a mixed form. More specifically, the"mixed region" means existence of the ferromagnetic powder originated inthe upper magnetic layer and the non-magnetic powder originated in thelower non-magnetic layer in the interface therebetween. Accordingly,with no mixed region, each of the ferromagnetic powder and thenon-magnetic powder undergoes no disturbance in orientation in thevicinity of the interface so that the interface in the cross section canbe traced without difficulty. As a result, surface properties of therecording layer are improved, leading to an increase in RF output and aneffective reduction in BER (block error rate) or drop-out. Further,thickness reduction of the magnetic layer to 1 μm or less leads tostable production of a magnetic recording medium suited for short waverecording.

While processes for producing the magnetic recording medium according tothe second embodiment are not particularly restricted, the means adoptedin the above-described exemplary means (E) through (G) can besuccessfully applied. That is, these magnetic recording media can beproduced by appropriately selecting the shape or size of theferromagnetic powder and/or non-magnetic powder, the kind of the binder,and the like.

In means (E), the lower non-magnetic layer contains a non-magneticpowder having an r₁ /r₂ ratio (i.e., axial ratio) of 2.5 or more. Thenon-magnetic powder having such a specific axial ratio is orientated inthe longitudinal direction through flow orientation on coating therebyinhibiting the rotary action thereof due to magnetic orientation of theferromagnetic powder. As a result, the disturbance at the interfacebetween the lower and upper layers, and also the orientation of theferromagnetic powder can be improved.

While "r₁ " and "r₂ " indicate the longest and shortest length of theparticle axis in average, respectively, the specific shape of thenon-magnetic powder is principally arbitrary and may be either acicularor tabular as observed under an electron microscope. The terminology"axis" as used herein does not mean an axis of symmetry in its strictsense. In the case of acicular particles, r₁ is usually called anaverage major axis length and is not more than 3 μm, and preferably notmore than 1.5 μm (in this case, r₂ corresponds to an average minor axislength or thickness), with a preferred axial ratio ranging from 5 to 20.In the case of flaky or tabular particles, r₁ corresponds to a so-calledplate diameter and ranges from 0.01 to 3 μm, and preferably from 0.05 to1.5 μm (in this case, r₂ is a thickness of the plate), with a preferredaspect ratio ranging from 5 to 20.

The non-magnetic powder is used in an amount usually of from 5 to 150%by weight and preferably from 10 to 100% by weight based on the binderin the lower non-magnetic layer.

While the ferromagnetic powder in the upper magnetic layer are notparticularly limited in physical properties, shape, and size, acicularor tabular ferromagnetic powders having the longest axis of not morethan 0.3 μm in average are preferred. Specific examples of the acicularferromagnetic powder are γ-Fe₂ O₃, Fe₃ O₄, Co-γ-Fe₂ O₃, CrO₂, andferromagnetic alloy powders (e.g., Fe--Ni and Fe--Ni--Co). Specificexamples of the tabular ferromagnetic powder include hexagonal ferritepowders, e.g., barium ferrite and strontium ferrite, and Co alloypowders. Particularly preferred are Fe-based alloys and hexagonalferrites.

In means (F), the upper magnetic layer has a ferromagnetic powder havingan average longest axis of not more than 0.3 μm, and the lowernon-magnetic layer contains a non-magnetic powder having an acicularratio of not less than 2.5. Differences from means (E) reside in thatthe shape of the non-magnetic powder is limited to an acicular form andthat the average longest axis of the ferromagnetic powder is limited to0.3 μm at the most, whereby the degree of powder packing and the outputcan be improved.

The terminology "acicular ratio" as used herein has the same meaning asr₁ /r₂ as defined in means (E) with respect to the non-magnetic powder.For making a distinction therefrom, the acicular ratio will hereinafterbe expressed as R₁ /R₂, wherein R₁ is the average longest axial length,and R₂ the average shortest axial length. R₁ is not more than 3 μm, andpreferably not more than 1.5 μm, with a preferred axial ratio being 5 ormore.

The acicular non-magnetic powder which can be used in means (F) includespowders of non-magnetic metals (e.g., Cu, Cr, Ag, Al, Ti, and W) oroxides thereof (e.g., α- or γ-Al₂ O₃, Cr₂ O₃, α-ferrite, goethite, SiO₂(inclusive of glass), ZrO₂, CeO₂, and rutile or anatase titaniumdioxide).

The acicular non-magnetic powder is usually used in an amount of from 5to 150% by weight and preferably from 10 to 100% by weight based on thebinder in the lower non-magnetic layer.

The lower non-magnetic layer generally has a thickness of not less than0.5 μm, and preferably between 0.5 and 5.0 μm. If it is thinner than 0.5μm, productivity tends to be reduced, and calender-moldability may bedeteriorated, failing to obtain sufficient electromagneticcharacteristics.

The ferromagnetic powder has an arbitrary shape and may be eitheracicular or tabular. The definition of the axial ratio r₁ /r₂ given inmeans (E) also applies to the average particle size of the ferromagneticpowder. For making a distinction therefrom, the axial ratio of theferromagnetic powder to be used in this means (F) will hereinafter beexpressed by φ₁ /φ₂. In the case of acicular powders, φ₁ is not morethan 0.3 μm, and preferably not more than 0.25 μm, with the φ₁ /φ₂ ratiobeing 2.5 or more. In the case of tabular powders, φ₁ is between 0.01and 0.3 μm, and preferably between 0.05 and 0.2 μm, with the φ₁ /φ₂ratio being 2.5 or more.

In means (G), the lower non-magnetic layer contains a flaky non-magneticpowder and a binder having an epoxy group as a functional group, and theupper magnetic layer contains an acicular or tabular ferromagneticpowder.

The flaky shape of the non-magnetic powder to be used in means (G) canbe defined by the same overall value range for axial ratio r₁ /r₂ usedfor the flaky powder used in means (E). However, for purpose of makingnecessary distinctions therefrom, the axial ratio of the flakynon-magnetic powder used in means (G) will hereinafter be expressed byr₃ /r₄. r₃ ranges from 0.1 to 5 μm, and preferably from 0.1 to 2 μm,with the r₃ /r₄ ratio preferably being 2.5 or more.

Specific but non-limiting examples of the flaky non-magnetic powder formeans (G) preferably include graphite, mica, and boron nitride.

The amount of the flaky non-magnetic powder added is generally from 10to 200% by weight and preferably from 12 to 120% by weight based on thebinder in the lower non-magnetic layer.

The shape of the ferromagnetic powder which can be used here is limitedto an acicular or tabular form, which can be defined by the same axialratio φ₁ /φ₂ as for the ferromagnetic powder used in means (F). Formaking a distinction therefrom, the axial ratio of the magnetic powderused in means (G) will hereinafter be expressed by φ₃ /φ₄. In the caseof acicular powders, φ₃ is not more than 0.3 μm, with the φ₃ /φ₄ ratiobeing not less than 2.5. In the case of tabular powders, φ₃ is not morethan 0.3 μm, with the φ₃ /φ₄ ratio being not less than 2.5.

Examples of suitable acicular ferromagnetic powders include γ-ironoxide, Co-doped iron oxide, CrO₂, and Fe-based alloy powders. Examplesof suitable tabular ferromagnetic powders include hexagonal ferriteferromagnetic powders (e.g., Ba ferrite, Sr ferrite), and Co alloypowders.

Conventional binders can be used for dispersing and binding theseferromagnetic powders in means (G). If desired, the magnetic coatingcomposition may further contain an abrasive, carbon black, a lubricant,and other conventional additives for magnetic powder containingcoatings.

Binders which can be used in the lower non-magnetic layer comprises atleast an epoxy-containing resin having a molecular weight of more than30,000 and preferably not more than 200,000. It appears that the epoxygroup of the binder resin is reacted with a hydroxyl group distributedon the surface of the non-magnetic powder thereby preventing stacking ofthe non-magnetic powder and improving dispersibility. Having a flakyshape, the non-magnetic powder is arranged in the lower non-magneticlayer in a configuration akin, to tiles covering a floor. Even when theferromagnetic powder in the upper magnetic layer coated thereonundergoes magnetic field orientation, it is under control by theso-arranged non-magnetic powder in the lower layer at the interface andthus suffers no mixing with the lower layer components, therebyaccomplishing satisfactory orientation. As a result, packing propertiesand surface properties are improved, and the RF output performance isincreased.

The epoxy group content in the epoxy-containing binder resin ispreferably in the range of from 1×10⁻⁵ to 20×10⁻⁴ eq/g, and morepreferably from 4×10⁻⁵ to 16×10⁻⁴ eq/g. Incorporation of an epoxy groupcan be carried out by conventional techniques. For example, anepoxy-containing resin can be prepared by copolymerizing a vinyl monomerhaving a glycidyl group and other monomers. In addition, the processhereinafter described with respect to an epoxy-containing vinyl chlorideresin can be used for the preparation of the epoxy-containing resinhaving a molecular weight of more than 30,000.

The epoxy-containing resin is generally used in an amount of from 5 to100% by weight and preferably from 10 to 70% by weight based on thetotal binder in the lower non-magnetic layer.

In means (G), it is preferable to add an abrasive having a Mohs hardnessof 5 or more to the lower non-magnetic layer at a non-magneticpowder/abrasive ratio of from 95/5 to 60/40 by weight. Incorporation ofsuch an abrasive is effective to increase the strength of the lowernon-magnetic layer, which leads to improved mechanical strength of theresulting magnetic recording medium. That is, the powder is preventedfrom falling off thereby to reduce BER and drop out and to improvedurability.

Examples of abrasives having a Mohs hardness of 5 or more include α-Al₂O₃, Cr₂ O₃, α-Fe₂ O₃, ZrO₂, TiO₂, TiC, SiO₂. SiC, and CeO₂. The particlesize of the abrasive is preferably not greater than the thickness of thelower non-magnetic layer, usually ranging from about 0.1 to 5μ, andpreferably from 0.1 to 2μ. The abrasive grain may have either a granularshape or an acicular shape. If the mixing ratio of the abrasive to theflaky non-magnetic powder is less than 5/95, sufficient durability maynot be obtained. If it is more than 40/60, the effects, of the flakypowder on orientation of the magnetic powder tend to be reduced.

In the magnetic recording medium satisfying the first feature of thepresent invention, layer (b) preferably has an average dry thickness (d)ranging from λ/4 to 3λ and a surface roughness (Ra) of not more thanλ/50.

To this effect, it is preferable that the ferromagnetic powder in layer(b) is an acicular powder having a major axis length of not more than0.3 μm or a tabular powder having a plate diameter of not more than 0.3μm and that the non-magnetic powder in layer (a) is a particulate powderhaving an average particle size of not more than λ/4 or an acicularpowder having a major (longest) axis length of from 0.05 to 1.0 μm andan acicular ratio of from 5 to 20, or a tabular powder having a platediameter of from 0.05 to 1.0 μm and an aspect ratio of from 5 to 20.

The above-mentioned surface properties can be achieved by the followingfour means (H) to (J) while controlling the standard deviation ofaverage dry thickness of layer (b) to 0.2 μm or less.

(H) The non-magnetic powder in layer (a) contains an inorganic powderhaving a Mohs hardness of 3 or higher, the ferromagnetic powder in layer(b) is an acicular powder, and said inorganic powder has an averageparticle size 1/2 to 4 times the crystallite size of said acicularferromagnetic powder.

(I) The non-magnetic powder in layer (a) contains an inorganic powderhaving a Mohs hardness of 3 or higher, the ferromagnetic powder in layer(b) is an acicular powder, and said inorganic powder has an averageparticle size not greater than 1/3 the major axis length of saidacicular ferromagnetic powder.

(J) The ferromagnetic powder in layer (b) is a hexagonal tabular powderhaving an axis of easy magnetization in the direction perpendicular tothe plate thereof, and the non-magnetic powder in layer (a) contains aninorganic powder having an average particle size of not more than theplate diameter of the ferromagnetic powder.

(K) The non-magnetic powder in layer (a) contains an inorganic powdercoated with an inorganic oxide.

The effects of means (H) are described below.

In order to coat layer (b) to such a very small thickness as 1 μm orless, simultaneous wet-on-wet coating is required. In this coatingsystem, surface roughness is decided by the relationship between theparticle size of the inorganic powder in layer (a) and the crystallitesize of the ferromagnetic powder in layer (b). In the case of acicularferromagnetic powders, a crystallite size approximately corresponds tothe shorter (minor) axis length. If the average particle size of theinorganic powder in layer (a) is less than 1/2 the crystallite size ofthe acicular ferromagnetic powder, dispersion of layer (a) becomesdifficult, failing to obtain a smooth surface, which failure leads toinsufficient surface smoothness of the finally obtained magneticrecording medium. To the contrary, if the average particle size of theinorganic powder in layer (a) exceeds 4 times the crystallite size ofthe ferromagnetic powder, the distance among particles in layer (a)becomes longer so that the ferromagnetic powder in layer (b) readilyundergoes influences of the surface properties of layer (a), resultingin a failure to obtain sufficient surface properties. As demonstrated inExamples hereinafter given, it is desirable, for assurance of sufficientsurface properties, to use an inorganic powder having an averageparticle size 1/2 to 4 times, and particularly 2/3 to 2 times, thecrystallite size of a ferromagnetic powder in layer (b). Such aninorganic powder preferably has a spherical shape or a cubic shape andhas a Mohs hardness of 3 or higher, preferably 4 or higher, and morepreferably 9 or higher.

Further, the inorganic powder in layer (a) preferably has a volumepacking of from 20 to 60%, and more preferably from 25 to 55%.

In order to reduce surface roughness by controlling the relationshipbetween the non-magnetic powder size and ferromagnetic powder size asdescribed above, there is a preferred range for the volume packing ofthe powder in layer (a). If the volume packing is less than 20%, thedistance among particles in layer (a) becomes longer so that theferromagnetic powder in layer (b) readily undergoes influences of thesurface properties of layer (a). Further, the ferromagnetic powder inlayer (b) is apt to be incorporated into layer (a), making the interfacerougher. Furthermore, the squareness ratio would be reduced. On theother hand, if the volume packing exceeds 60%, the coating compositionhas too a high viscosity for coating. If coating may be carried out, theresulting medium would have poor running durability, suffering fall-offof powders.

The above-described inorganic powder is preferably present in an amountof 60% by weight or more based on the total non-magnetic powder. Theinorganic powder to be used preferably includes metal oxides andalkaline earth metal salts. It is preferable to use carbon black incombination with the inorganic powder with the expectation of obtainingknown effects, for example, reduction in surface resistivity. However,having a very poor dispersibility, carbon black alone is not sufficientfor obtaining satisfactory electromagnetic characteristics. Forsatisfactory dispersibility, at least 60% by weight of total inorganicpowder must be selected from metal oxides, metals, and alkaline earthmetal salts. Should the proportion of the inorganic powder be less than60%, with that of carbon black exceeding 40% by weight, dispersibilitywould be insufficient for obtaining desired electromagneticcharacteristics.

The effects of means (I) are described below.

For maintaining satisfactory electromagnetic characteristics bywet-on-wet coating, it is necessary to increase a squareness ratio. Ifthe inorganic powder in layer (a) has a large average particle size withrespect to the magnetic powder in layer (b), the distance amongparticles in layer (a) becomes large to cause disturbance of orientationof the ferromagnetic powder particularly at the interface between layers(a) and (b), resulting in deterioration of surface properties of layer(b) as discussed above with respect to means (H). In order to reducesuch orientation disturbance, it is required that fine non-magneticparticles should be arranged along the longer axis direction of theferromagnetic powder to support the ferromagnetic particles so as toprevent orientation disturbance over the longitudinal direction of theferromagnetic powder. In using an acicular ferromagnetic powder in layer(b), experimentation proved that use of an inorganic powder whoseaverage particle size is not more than 1/3, preferably of from 1/3 to1/2, the major axis length of the acicular ferromagnetic powder in layer(a) realizes a squareness ratio equal to that of a single magnetic layerand satisfactory surface properties.

The same approach is taken in means (J). That is, use of a hexagonaltabular ferromagnetic powder in place of an acicular ferromagneticpowder brings about vertical orientation to suppress the interfacialdisturbance thereby increasing the squareness ratio. The averageparticle size of the inorganic powder to be used in layer (a) should notbe greater than the plate diameter of the hexagonal tabular powder, andis preferably not less than 1/5 the plate diameter.

In means (I) and (J), the volume packing of the inorganic powder inlayer (a) preferably ranges from 20 to 60% for the same reasons asdescribed in (H).

Further, by controlling the thickness of layer (b) below 5 times themajor axis length of the ferromagnetic powder, the effect of calenderingon improvement of packing is enhanced to further improve electromagneticcharacteristics.

Preferred kinds and properties of the inorganic powder to be used inmeans (I) and (J) are the same as described with respect to means (H).

The effects of means (K) are as follows. Examples of suitable inorganicoxides to be coated on the inorganic powder in layer (a) include Al₂ O₃,SiO₂, TiO₂, ZrO₂, SnO₂, Sb₂ O₃, and ZnO, with Al₂ O₃, SiO₂, and ZrO₂being particularly preferred. These inorganic oxides may be used eitherindividually or in combinations thereof. If desired, surface treatmentof the inorganic powder may be carried out by co-precipitation of theabove-mentioned oxide compounds, or the inorganic powder is firsttreated with alumina and then with silica, or vise versa. The surfacetreating coat may be porous depending on the end use, but is, ingeneral, preferably has a uniform and dense structure.

The surface treatment of the non-magnetic inorganic powder can beeffected, for example, as follows. A material(s) of the non-magneticinorganic powder is dry ground and then wet ground together with waterand a dispersing agent, followed by centrifugal separation for removingcoarse particles. The resulting finely divided slurry is transferred toa surface treatment bath where the dispersed particles are coated with ametal hydroxide. A prescribed amount of an aqueous solution of a salt ofAl, Si, Ti, Zr, Sb, Sn, Zn, etc. is added to the bath, and aneutralizing acid or alkali is added thereto to form a hydrous oxidewith which the inorganic powder is coated. The by-produced water-solublesalt is removed by decantation, filtration, or washing, and the slurryis finally adjusted to a prescribed pH, followed by filtration andwashing with pure water. The washed filter cake is dried by a spraydrier or a band drier. The dried product is pulverized in a jet mill.The surface treatment with Al or Si may also be carried out by applyingvapors of AlCl₃ or SiCl₄ to the non-magnetic inorganic powder and thenpassing steam therethrough.

With respect to other suitable surface treatments, reference can be madeto Characterization of Powder Surfaces, Academic Press.

In the above-described embodiments in which the range of thickness oflayer (b) optimum for a particular recording wavelength and the upperlimit of thickness variation of layer (b) (i.e., a width of variation inthe thickness direction) are specified, the surface roughness of layer(b) is decided with an improvement, leading to a success of forming athin, uniform, and even magnetic layer. As a result, high reproductionoutput and high C/N ratio can be reached while preventing variations inreproduction output and amplitude modulation noise even if the recordingwavelength becomes shorter. In the conventional magnetic recording mediawith the thickness of the magnetic layer being reduced, if the recordingwavelength becomes shorter, variations in thickness of the magneticlayer (contributing to reproduction in its totality) caused variationsin reproduction output and amplitude modulation noise. Suchdisadvantages of the conventional techniques can thus be overcome by thepresent invention.

The shortest recording wavelength λ varies depending on the type ofmagnetic recording media. For example, it is 0.7 μm for 8 mm metal videotapes, 0.5 μm for digital video tapes, or 0.67 μm for digital audiotapes.

The thickness d of layer (b) according to the present invention is inthe range of λ/4≦d≦3λ, and preferably λ/4≦d≦2λ (i.e., 0.25≦d/λ≦2). Theaverage thickness d of layer (b) according to the present inventionusually falls within a range of from 0.05 to 1 μm, and preferably from0.05 to 0.8 μm.

The thickness of layer (b) can be obtained by actual measurements asdescribed above. It is also obtainable by fluorescent X-ray method. Inthis case, a calibration curve of fluorescent X-ray intensity for anelement inherently contained in a magnetic layer is prepared frommagnetic layer samples having a known thickness, and a thickness of asample of unknown thickness is obtained from its fluorescent X-rayintensity.

In the present invention, .sup.Δ d is controlled below d/2, that is,.sup.Δ d/d is controlled below 0.5, preferably below 0.3, and morepreferably below 0.25. .sup.Δ d ranges from 0.001 to 0.5 μm, preferablyfrom 0.03 to 0.3 μm, and more preferably from 0.05 to 0.25 μm. Withthese conditions being fulfilled, layer (b) is assured of evenness ofits thickness and, at the same time, has its surface roughness Racontrolled at or below λ/50, i.e., to control λ/Ra at or above 50,preferably at or above 75, and more preferably at or above 80. Theterminology "surface roughness Ra" as used herein means a centerlineaverage roughness as measured with an interference roughness tester.

The magnetic recording medium of the present invention are produced byforming layer (b) on layer (a) either by simultaneous coating orsuccessive coating while layer (a) is wet. Simultaneous wet-on-wetcoating is preferred. The magnetic recording medium produced by thewet-on-wet coating system of the present invention, for example, bysimultaneous coating of a lower non-magnetic layer and a 1 μm or lessthick upper magnetic layer is free from various coating defects whichdevelop with a single magnetic layer having a reduced thickness or witha double-layer structure formed by coating an upper magnetic layer on adried lower non-magnetic layer (so-called wet-on-dry coating).

The inventors ascertained that mere reduction in magnetic layerthickness is not enough and that there is an optimum thickness range inrelation to the shortest recording wavelength λ. That is, if themagnetic layer thickness is thinner than λ/4, a magnetic fluxcontributing to reproduction decreases to cause reduction of output. Ifit exceeds 3λ, short wavelength components undergo demagnetization by adeep layer recording magnetic field of simultaneously recorded longwavelength components, which also causes reduction of output.Accordingly, d≦3λ, and preferably d≦2λ.

Turning to a C/N, a fundamental factor of performance properties ofmagnetic recording media, it is influenced by not only unevenness of amagnetic layer surface (surface roughness), which is a conventionallyidentified problem with a thick magnetic layer, but also a thicknessvariation at the interface between a non-magnetic layer and a magneticlayer. The latter consideration was not important in the conventionalthick magnetic layer but carries weight with the magnetic layerthickness d falling within the specific range λ/4≦d≦3λ. This is becausean output is influenced by the magnetic flux of the whole magnetic layerwith d falling within the above range. In this connection, it was foundthat an average thickness variation .sup.Δ d at the interface betweenlayers (a) and (b) should be not more than d/2. The surface of layer (b)is required to be smooth as in the conventional thick magnetic layer,and surface roughness Ra is required to satisfy the relationship:

    Ra≦λ/50.

The above-described embodiments for the first feature thus make itpossible to produce high performance coated type magnetic recordingmedia exhibiting electromagnetic characteristics comparable to thinmetal tapes at high productivity without being accompanied by theproblems associated with metal tapes, such as necessity of treatment invacuum, susceptibility to corrosion, and low productivity.

The second feature of the present invention is that the squared meansurface roughness R_(rms) of layer (b) as measured with a scanningtunnel microscope (STM) and an average dry thickness d of layer (b)satisfies a relationship:

    30≦d/R.sub.rms.

As the thickness of a magnetic layer is reduced, self-demagnetizationloss is expected to be reduced to increase output. At the same time,however, reduction of thickness means reduction in margin to be pressedon calendering. It follows that the effect of calendering is lessened,resulting in increased surface roughness. In order to decreaseself-demagnetization loss to thereby improve output, it is preferable tocontrol the STM surface roughness so as to satisfy the aboverelationship.

R_(rms) as measured with an atomic force microscope (AFM) is preferablynot more than 10 nm. Interference surface roughness Ra as measured with3d-MIRAU is preferably from 1 to 4 nm, and a peak-to-valley value (P-Vvalue) is preferably not more than 80 nm.

Gloss of the surface of layer (b) after calendering is preferably from250 to 400%.

The third feature of the present invention can be accomplished by means(L) or (M):

(L) The ferromagnetic layer in layer (b) is an acicular ferromagneticalloy powder having a major axis length of not more than 0.3 μm and acoercive force (Hc) of not less than 1500 Oe or a tabular ferromagneticpowder having a plate diameter of not more than 0.3 μm and an Hc of notless than 1000 Oe.

(M) Layer (b) contains a ferromagnetic alloy powder having a major axislength of not more than 0.25 μm and an acicular ratio of not more than12, and layer (b) has an Hc of not less than 1500 Oe in the longitudinaldirection and not less than 1000 Oe in the width direction.

According to means (L) and (M), variation or disturbance at theinterface between layers (a) and (b) can be suppressed to preventorientation disturbance of the ferromagnetic powder in layer (b),whereby the coercive force of layer (b) having a dry thickness of notmore than 1.0 μm in each of normal direction, longitudinal direction,and width direction can be set above a certain level to improveelectromagnetic characteristics particularly in short wavelengthrecording.

Means (L) is described below in detail.

Residual coercive force (Hr) as used herein means a value obtained asfollows. A magnetic field of 10 kOe is applied to a magnetic recordingmedium sample in the in-plane direction of layer (b). When the thusmagnetized sample is turned 90° in the thickness direction, theintensity of the outer magnetic field applied in the normal directionwith respect to layer (b) which is required for making the residualmagnetization zero is a residual coercive force.

According to means (L), Hr is set at 1500 Oe or higher, by whichdemagnetization loss during recording can effectively be inhibited toattain high output. This is probably because Hr corresponds to avertical magnetization component. Because the contribution of thecoercive force of this vertical magnetization component increasesparticularly in short wavelength recording, it is believed that settingHr high brings about inhibition of demagnetization loss on recording.

Means (L) can be realized by proper selection of an inorganic powder tobe used in layer (a) for the purpose of achieving high Hr therebyminimizing orientation disturbance of the ferromagnetic powder at theinterface between layers (a) and (b) and also by using a ferromagneticpowder having large magnetization anisotropy (i.e., having a higher Hcin one direction) and preferably by conducting orientation in multiplestages.

Means (L) thus makes it possible to produce high performance coated typemagnetic recording media exhibiting electromagnetic characteristicscomparable to thin metal tapes at high productivity without beingaccompanied by the problems associated with metal tapes, such asnecessity of treatment in vacuum, susceptibility to corrosion, and lowproductivity.

While the ferromagnetic powders capable of assuring a high Hr are notparticularly limited in kind, proper choice of the shape and kindthereof in carrying out means (L) produces greater effects.

That is, the ferromagnetic powder which can be used in means (L) is anacicular ferromagnetic alloy powder having a major axis length of notmore than 0.3 μm, preferably not more than 0.25 μm, and a coercive force(Hc) of not less than 1500 Oe, preferably not less than 1550 Oe, or atabular ferromagnetic powder having a plate diameter of not more than0.3 μm, preferably not more than 0.2 μm, and an Hc of not less than 1000Oe, preferably not less than 1200 Oe.

The above-mentioned acicular ferromagnetic alloy powder, inclusive of asingle metal powder, preferably has an acicular ratio (longest axislength/shortest axis length) of from 2 to 15, and preferably from 5 to12. Specific examples of such an acicular ferromagnetic alloy powderinclude single metals, e.g., Fe, Ni, and Co; and alloys comprising sucha metal as a main component (at least 75%) and other prescribedelements. If desired, other elements, e.g., Al, Si, S, Sc, Ti, V, Cr,Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi, La,Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, and B, may further be present for thepurpose of improving various characteristics.

The tabular ferromagnetic powder to be used in means (L) has an aspectratio (plate diameter/thickness ratio) of from 2 to 30, and preferablyfrom 5 to 20. Examples of such tabular ferromagnetic powders arehexagonal ferrite ferromagnetic powders (e.g., barium ferrite, strontiumferrite, lead ferrite, calcium ferrite, cobalt ferrite), and hexagonalCo powder. More specifically, barium ferrite or strontium ferrite ofmagnetoplumbite type and barium ferrite or strontium ferrite ofmagnetoplumbite type locally containing a spinel phase are included.Particularly preferred are barium ferrite and strontium ferrite. Inorder to control coercive force, the above-mentioned hexagonal ferritehaving added thereto Co--Ti, Co--Ti--Zr, Co--Ti--Zn, Ni--Ti--Zn, Ir--Zn,etc. can be used. The plate diameter is a width of a tabular particleand can be measured under electron microscopic observation.

In means (L), ferromagnetic powders other than those enumerated above,e.g., magnetic iron oxide FeO, (x=1.33 to 1.5) and Co-doped FeO, (x=1.33to 1.5), may also be used either alone or in combination with theabove-mentioned ferromagnetic powders.

The inorganic powders which can be used in layer (a) in means (L)include metals (e.g., Cu, Cr, Ag, Al, Ti, and W), metal oxides, metalcarbonates, metal sulfates, metal nitrides, metal carbides, and metalsulfides. Specific examples include TiO₂ (rutile or anatase), TiO_(x),cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO₂, SiO₂, Cr₂ O₃,α-alumina (α ratio: 90% or more), β-alumina, γ-alumina, α-iron oxide,goethite, corundum, silicon nitride, titanium carbide, magnesium oxide,boron nitride, molybdenum disulfide, copper oxide, MgCO₃, CaCO₃, BaCO₃,SrCO₃, BaSO₄, silicon carbide, and titanium carbide, either alone or incombination of two or more thereof. The shape, size, and the like ofthese inorganic powders are arbitrary. Two or more different kinds ofinorganic powders may be combined, or a single kind of the inorganicpowder having a selected size distribution may be employed according tothe purpose sought. Particulate inorganic powders usually have aparticle size of not more than 0.2 μm, and preferably between 0.005 and0.08 μm, and acicular inorganic powders usually have a major axis lengthof from 0.05 to 1.0 μm, and preferably from 0.05 to 0.5 μm, with anacicular ratio ranging from 5 to 20, and preferably from 5 to 15.

From the standpoint of physical properties, preferred inorganic powdersare those having a tapped density of from 0.05 to 2 g/cc, and preferablyfrom 0.2 to 1.5 g/cc, a water content of from 0.1 to 5%, and preferablyfrom 0.2 to 3%, a pH of from 2 to 11, and a DBP absorption of from 5 to100 ml/100 g, preferably from 10 to 80 ml/100 g, and more preferablyfrom 20 to 60 ml/100 g. The non-magnetic inorganic powder does not needto be 100% by weight pure and may be surface-treated with othercompounds, e.g., compounds of Al, Si, Ti, Zr, Sn, Sb, Zn, etc., to formthereon a coat of an oxide of such element according to the purposesought. In this case, a purity of 70% by weight would be enough. Anignition loss of the inorganic powder is preferably not more than 20% byweight.

These inorganic powders are commercially available under trade names ofUA 5600 and US 5605 (both produced by Showa Denko K.K.); AKP-20, AKP-30,AKP-50, HIT-50, HIT-100, and ZA-G1 (all produced by Sumitomo ChemicalCo., Ltd.); G5, G7, and S-1 (all produced by Nippon Chemical IndustrialCo., Ltd.; TF-100, TF-120, TF-140, R 516 (all produced by Toda KogyoK.K.); TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, FT-1000,FT-2000, FTL-100, FTL-200, M-1, S-1, SN-100, R-820, R-830, R-930, R-550,CR-50, CR-80, R-680, and TY-50 (all produced by Ishihara Sangyo Kaisha,Ltd.); ECT-52, STT-4D, STT-30D, STT-30, and STT-65C (all produced byTitan Kogyo K.K.); T-1 (produced by Mitsubishi Material Corp.); NS-O,NS-3Y, and NS-8Y (all produced by Nippon Shokubai Kagaku Kogyo Co.,Ltd.); MT-100S, MT-100T, MT-150W, MT-500B, MT-600B, and MT-100E (allproduced by Teika K.K.); FINEX-25, BF-1, BF-10, BF-20, BF-1L, and BF-10P(all produced by Sakai Kagaku K.K.); DEFIC-Y and DEFIC-R (both producedby Dowa Kogyo K.K.); and Y-LOP and calcined product thereof (produced byTitan Kogyo K.K.).

A particularly preferred non-magnetic inorganic powder for use in thepresent invention is titanium oxide, and especially titanium dioxide.Process for producing titanium oxide is described below in detail.Preparation of titanium oxide chiefly includes a sulfuric acid processand a chlorine process. According to the former process, a raw ore ofilmenite is distilled in sulfuric acid to extract Ti, Fe, etc. in theform of a sulfate, and iron sulfate is removed by crystallization. Afterpurifying the residual titanium sulfate solution by filtration,hydrolysis is conducted under heating to precipitate hydrous titaniumoxide, which is then filtered and washed to remove impurities. Aparticle size regulator, etc. is added thereto, followed by calcinationat 80 to 1000° C. to obtain crude titanium oxide. The structure type,rutile or anatase, is decided by the kind of a nucleating agent added onhydrolysis. The resulting crude titanium oxide is subjected to finishingtreatments, such as pulverization, classification, surface treatment,and so on. The chlorine process is applied to a naturally-occurringrutile type ore or synthetic rutile. An ore is chlorinated in a reducedstate at a high temperature, whereby Ti is converted to TiCl₄, and Fe toFeCl₂. After cooling, solidified iron oxide is separated from liquidTiCl₄. The resulting crude TiCl₄ is purified by rectification and, afteraddition of a nucleating agent, brought into an instantaneous reactionwith oxygen at 1000° C. or higher to obtain crude titanium oxide, whichis then subjected to the same finishing treatments as described abovefor imparting pigment properties.

In the present invention, layer (a) may further contain carbon black todecrease surface resistivity R_(s) as commonly expected. Carbon blackwhich can be used in the present invention includes furnace black forrubbers, thermal black for rubbers, color black, and acetylene black.Carbon black to be used preferably has a specific surface area of from100 to 500 m² /g, a DBP absorption of from 20 to 400 ml/100 g, anaverage particle size of from 5 to 30 mμ, a pH of from 2 to 10, a watercontent of from 0.1 to 10%, and a tapped density of from 0.1 to 1 g/cc.

Specific examples of commercially available carbon black which can beused in the present invention are BLACKPEARLS 2000, 1300, 1000, 900,800, 880 and 700 and VULCAN XC-72 (all produced by Cabot Corp.); #3050,#3150, #3250, #3750, #3950, #2400B, #2300, #1000, #970, #950, #900,#850, #650, #40, MA 40, and MA-600 (all produced by Mitsubishi ChemicalIndustries, Ltd.); CONDUCTEX SC (produced by Columbian Co., Ltd.); 8800,8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, and 1250(all produced by Raven); and Ketjenblack EC (produced by Akizo ChemieNederland B.V.). Carbon black having been surface treated with adispersing agent, etc., resin-grafted carbon black, or carbon blackhaving its surface partly graphitized may be employed. Carbon black maypreviously be dispersed in a binder before it is added to a magneticcoating composition. The above-described carbon black species may beused either individually or in combination of thereof. In choosingcarbon black species for use in the present invention, Carbon BlackKyokai (ed.), Carbon Black Binran can be referred to.

If desired, layer (a) may furthermore contain a non-magnetic organicpowder, such as an acrylate-styrene copolymer resin powder, abenzoguanamine resin powder, a melamine resin powder, a phthalocyaninepigment, a polyolefin resin powder, a polyester resin powder, apolyamide resin powder, a polyimide resin powder, and apolyfluoroethylene resin powder. Processes for producing these organicpowders are described, e.g., in JP-A-62-18564 and JP-A-60-255827.

The total non-magnetic powder is usually used at a weight ratio tobinder of from 20 to 0.1, and at a volume ratio to binder of from 10 to0.1.

While a subbing layer is generally provided in magnetic recording media,it is different from the lower non-magnetic layer according to thepresent invention in not only thickness (generally 0.5 μm or less) butthe purpose sought (for improvement of adhesion between a support and anupper layer, e.g., a magnetic layer). Provision of a subbing layer isalso recommended in the present invention for improving adhesion betweena support and layer (a).

Multi-staged orientation of layer (b) can be carried out by anyconventional method. For example, layer (b) is orientated while bothlayers (a) and (b) are wet with a cobalt magnet of 3000 G and then witha solenoid of 1500 G.

Means (M) is illustrated below in detail.

The,, terminology "coercive force Hc in the longitudinal direction oflayer (b)" means the outer magnetic field applied in parallel with layer(b) and in the longitudinal direction of layer (b) which is necessary tobring a residual magnetic field to zero after applying a magnetic fieldof 10 kOe to a sample in its in-plane direction with a vibrating samplemagnetometer (VSM), the "longitudinal direction" being a directionparallel with the in-plane and with the coating orientation direction.Likewise, the terminology "coercive force Hc in the width direction oflayer (b)" means the outer magnetic field applied in parallel with layer(b) and in the width direction of layer (b) which is necessary to bringa residual magnetic field to zero after applying a magnetic field of 10kOe to a sample in parallel with its in-plane direction and in its widthdirection with VSM, the "with direction" being a direction parallel withthe in-plane and perpendicular to the coating orientation direction.

Means (M) is characterized by setting Hc in the longitudinal directionof a magnetic layer having a thickness of not more than 1.0 μm at 1500Oe or higher, and preferably 1600 Oe or higher, and setting Hc in thewidth direction at 1000 Oe or higher, and preferably 1100 Oe or higher,thereby improving electromagnetic characteristics, particularlyreproduction output, while inhibiting crosstalk to reduce BER.

The effects of action of means (M) are the same as those described withrespect to means (L), with the vertical magnetization component asreferred to in means (L) being replaced with a magnetization componentin the longitudinal or width direction. That is, means (M) aims atinhibition of recording demagnetization of a magnetization component inthe longitudinal or width direction. The problem of productivityassociated with the conventional techniques in attempting to obtain highperformance properties of a magnetic layer can be resolved by using aninorganic powder in layer (a). While embodiments of layer (a) are notparticularly restricted in means (M), it is preferable to make a properselection of the inorganic powder to be incorporated into layer (a) andto control the thickness of layer (a) as described below.

Further, in means (M), a ferromagnetic alloy powder having a major axislength of not more than 0.25 μm, and preferably not more than 0.2 μm,and an acicular ratio of not more than 12, and preferably of from 7 to11, is employed for assurance of Hc in both longitudinal and widthdirections. The ferromagnetic alloy powders to be used are the same asthe acicular ferromagnetic alloy powders enumerated above with referenceto means (L).

The inorganic powder which can be used in means (M) preferably includesparticulate particles having a true specific gravity of not less than 3,and preferably between 4 and 8, and an average particle size of not morethan 0.2 μm, and preferably of from 0.005 to 0.08 μm, and acicularparticles having a true specific gravity of not less than 3, andpreferably from 4 to 8, a major axis length of from 0.05 to 1.0 μm, andpreferably from 0.05 to 0.5 μm, and an acicular ratio of from 5 to 20,and preferably from 5 to 15. The same specific examples of the inorganicpowders as enumerated above for means (L) apply to means (M). Theterminology "true specific gravity" as used herein has the same meaningas true density, a substantial density excluding any pores or voidsamong particles.

A preferred thickness of layer (a) is 0.5 μm or greater, andparticularly from 0.5 to 5.0 μm. It the thickness of layer (a) is lessthan 0.5 μm, productivity tends to fall, and calender effects tend to bereduced, causing a failure of obtaining sufficient electromagneticcharacteristics.

Other factors, such as carbon black and non-magnetic organic powders tobe incorporated into layer (a), are the same as described in means (L).

Further, layer (b) of the magnetic recording medium fulfilling the thirdfeather of the present invention has a standard deviation 3σ of not morethan 0.6 μm, and preferably not more than 0.2 μm. That is, it would beenough if 97% of segments fall within 0.6 μm. 3σ is preferably not morethan 6d/10.

Standard deviation a can be obtained as follows. A magnetic recordingmedium is sliced along its longitudinal direction with a diamond cutterto a thickness of about 0.1 μm. Micrographsm (print size: A4 to A5) ofthe cut area were taken with a transmission type electron microscope(TEM) at a magnification of 10000 to 100000, preferably 20000 to 50000.The interface between layers (a) and (b) was identified with the nakedeye with attention paid to a difference in shape between the magneticpowder in layer (b) and the non-magnetic powder in layer (a) and tracedwith a black marker pen. The surface of layer (b) is also traced with ablack marker pen. By the use of an image analyzer "IBAS 2" manufacturedby Zeiss Co., the distance in the thickness direction between the twoblack lines is measured in each of 100 to 300 divided segments to obtaina standard deviation according to the following formula, taking layer(b) thickness in each segment as x₁ : ##EQU1##

The aforementioned .sup.Δ d gives a consideration of only theinterfacial variation, while standard deviation a of average thickness dgives a consideration of layer (b) thickness variation inclusive of bothsurface roughness of layer (b) and the interfacial variation. Theinterfacial variation is preferably such that 3σ is within 0.6 μm.

Thus, surface roughness Ra can be controlled so as to satisfy arelationship: Ra≦λ/50, i.e., λ/Ra≧50, preferably ≧75, and morepreferably ≧100, while maintaining evenness of the layer (b) thickness(Ra: centerline average roughness measured with interference roughnesstester).

Further, the thickness d and surface roughness Ra of layer (b) arepreferably related with the shortest recording wavelength λ so as tohave relationships: λ/4≦d≦3λ, preferably λ/4≦d≦2λ (i.e., 0.25≦/λd≦2);and Ra≦λ/50, respectively.

There is thus obtained a magnetic recording medium having a highreproduction output and a high C/N ratio while preventing reproductionoutput variations or amplitude modulation noise.

The above-described surface characteristics can be accomplished by means(H) to (J) above discussed.

The fourth feature of the present invention can preferably be realizedby a means in which the stiffness of the magnetic recording medium iscontrolled so as to have an SMD/STM ratio of from 1.0 to 1.9.

To this effect, it is preferable to use a spherical to cubic polyhedralinorganic powder having a Mohs hardness of 6 or higher and an averageparticle size of not more than 0.15 μm in layer (a). With an SMD/STDratio being so adjusted, dynamic characteristics of the magneticrecording medium are controlled to have improved contact with a headwhile improving electromagnetic characteristics in short wavelengthrecording.

SMD and STD can be measured with a commercially available stiffnesstester, e.g., a loop stiffness tester manufactured by Toyo Seiki K.K. A8 mm wide and 50 mm long specimen is cut out of a magnetic recordingmedium sample in such a manner that the lengthwise direction agrees withthe coating direction of the sample (for measurement of SMD) or with thewidth direction of the sample (for measurement of STD), and both ends ofthe specimen are adhered to make a loop. A load is applied to the innerdiameter direction at a rate of displacement of 3.5 mm/sec, and a load(mg) required for giving a displacement of 5 mm is taken as SMD or STD.

The SMD/STD ratio is controlled within α-range of from 1.0 to 1.9, andpreferably from 1.1 to 1.85. A magnetic recording medium having a totalthickness of 13.5±1 μm has an SMD of from 50 to 200 mg, and preferablyfrom 50 to 150 mg, and an STD of from 40 to 150 mg, and preferably from50 to 130 mg.

Means for SMD/STD control is not particularly limited, but the controlis preferably effected by selection of the shape and Mohs hardness ofthe inorganic powder to be used in layer (a). The inorganic powder ispreferably selected from spherical to cubic, polyhedral inorganicpowders having a Mohs hardness of 6 or more, and particularly 6.5 ormore, and an average particle size of not more than 0.15 μm, andparticularly not more than 0.12 μm.

In order to obtain satisfactory contact with a head, the magneticrecording medium is required to have increased stiffness in eachdirection to some extent. To this effect, the powder to be usedpreferably has hardness. If Mohs hardness is less than 6, each stiffnessdecreases, failing to retaining satisfactory contact with a head.Further, a small average particle size of 0.15 μm or less is good forsatisfactory contact with a head probably because the contact area witha binder is so increased that both SMD and STD would be improved themore. In the present invention, the SMD/STD ratio is adjusted within theabove-recited specific range.

Particularly excellent effects on electromagnetic characteristics aremanifested as STD approaches SMD, i.e., the SMD/STD ratio approaches 1.In this connection, use of a polyhedral inorganic powder, inclusive offrom a spherical powder to a cubic powder, in layer (a) makes dynamicphysical properties of the coating film isotropic, which is advantageousfor improving STD.

The polyhedral inorganic powders include spherical powders and regularor irregular polyhedral powders having one or more regular or irregularpolygons such as regular n-gons (e.g., a regular square, a regularpentagon, and a regular hexagon) and irregular n-gons. Those having anaxial ratio of arbitrarily selected two axes ranging from 0.6 to 1.4,and preferably from 0.7 to 1.3, are desirable.

The fourth feature of the present invention can also be realized byanother means in which a percent thermal shrinkage of the magneticrecording medium at 80° C.×30 minutes has is controlled not to exceed0.4%. To this effect, layer (a) has a dry thickness 1 to 30 times thedry thickness of layer (b), and a difference subtracting a powder volumeratio of layer (b) from that of layer (a) is within a range of from -5%to +20%; the ferromagnetic powder in layer (b) has a crystallite size ofnot more than 300 Å, and the inorganic powder in layer (a) is aparticulate powder having an average particle size of less than 0.15 μmor an acicular powder having an average major axis length of less than0.6 μm; the inorganic powder in layer (a) is at least one of titaniumoxide, barium sulfate, calcium carbonate, strontium sulfate, silica,alumina, zinc oxide, and α-iron oxide; and layer (a) further contains,as a second component, less than 50 parts by weight of carbon blackhaving an average particle size of not more than 30 mμ, a DBP absorptionof from 30 to 300 ml/100 g, and a BET specific surface area of from 150to 400 m² /g per 100 parts by weight of the above-described inorganicpowder.

This means aims at production of a coated type magnetic recording mediumhaving its magnetic layer thickness reduced to 1 μm or less to reduceself-demagnetization loss without causing coating defects such aspinholes and streaks and also at controlling thermal shrinkage of themedium below a certain level.

By controlling percent thermal shrinkage after preservation at 70° C.for 48 hours to 0.4% or less, skewness can be diminished while obtainingexcellent electromagnetic characteristics comparable to ferromagneticmetal thin films.

In other words, this means is based on the finding that a properstrength of a magnetic recording medium with which a magnetic recordingmedium having a very thin magnetic layer can be produced with goodproductivity while minimizing skewness can be specified by theabove-described percentage thermal shrinkage.

The terminology "percent thermal shrinkage" as used herein means a valuecalculated by 100×[(length of magnetic recording medium at roomtemperature before heating)-(length of magnetic recording medium afterpreserved at 70° C. for 48 hours without tension)]/(length of magneticrecording medium before heating).

Means for controlling percent thermal shrinkage are not particularlyrestricted. More specifically, the following means are suitable.

The dry thickness of layer (a) is made 1 to 30 times, preferably 2 to 20times, that of layer (b), whereby expansion and contraction of themagnetic recording medium are controlled by film strength of layers (a)and (b). If the layer (a) thickness is less than the layer (b)thickness, the film strength is not sufficient for preventing thermalshrinkage from increasing with a reduction in layer (b) tihckness. Ifthe layer (a) thickness is more than 30 times the layer (b) thickness,the total thickness of the recording medium increases to increase aresidual solvent, which leads to disadvantages such as plasticization ofthe film.

The film strength of layers (a) and (b) can be properly adjusted bycontrolling a difference subtracting a powder volume ratio of layer (b)from that of layer (a) within a range of from -5% to +20%, andpreferably from 0 to 15%. If this difference is less than -5%, anincrease in thermal shrinkage of layer (b) cannot be suppressed. If itexceeds 20%, the magnetic recording medium itself becomes too hard,causing fall-off of the powder.

The powder volume ratio of layer (b) usually ranges from 10 to 50%, andpreferably from 20 to 45%, and that of layer (a) usually ranges from 20to 60%, and preferably from 25 to 50%.

The powder volume ratio of each layer can be controlled by adjustment ofpowder to binder ratio and size and shape of the powders in each layer.An increase in binder amount results in a relative decrease in powdervolume ratio. The finer the powder, the greater the effect on reductionof percent thermal shrinkage. However, too fine powders are difficult todisperse.

The ferromagnetic powder to be used in this means preferably has acrystallite size of not more than 300 Å, and particularly from 100 to250 Å, and an average major axis diameter of from 0.005 to 0.4 μm, andparticularly from 0.1 to 0.3 μm, with an average major axisdiameter/crystallite size ratio being from 3 to 25, and particularlyfrom 5 to 20. The ferromagnetic powder preferably has a BET specificsurface area of from 25 to 80 m² /g, and particularly from 30 to 70 m²/g. If the BET specific surface area is less than 25 m² /g, noise tendsto increase. If it exceeds 80 m² /g, desired surface properties arehardly obtained.

The inorganic powder in layer (b) to be used in this means preferablyincludes a particulate powder having an average particle size of lessthan 0.15 μm, and particularly from 0.005 to 0.7 μm, and an acicularpowder having an average major axis length of less than 0.6 μm, andparticularly from 0.1 to 0.3 μm, and an acicular ratio (average majoraxis length/minor axis length) of from 4 to 50, and particularly from 5to 30.

The inorganic powders to be used preferably include rutile titaniumoxide, α-iron oxide, and goethite.

Layer (b) may further contain carbon black. Carbon black to be usedusually has an average particle size of not more than 30 mμ, andpreferably from 5 to 28 mμ, a DBP absorption of from 30 to 300 ml/100 g,and preferably from 50 to 250 ml/100 g, a BET specific surface area offrom 150 to 400 m² /g, and preferably from 170 to 300 m² /g, a pH offrom 2 to 10, a water content of from 0.1 to 10%, and a tapped densityof from 0.1 to 1 g/cc.

Carbon black is preferably used in an amount of less than 50 parts byweight, and more preferably from 13 to 40 parts by weight, per 100 partsby weight of the above-described inorganic powder.

Carbon black functions to prevent static charge and to enhance filmstrength. It also serves for void volume control. Since a high voidvolume results in a relative decrease of powder volume ratio, carbonblack is useful for controlling powder volume ratio of layer (a). Inparticular, carbon black particles having a structure or hollow carbonblack particles are effective for this purpose. A void volume of layer(a) preferably falls within ±10% of that of layer (b) and preferablyranges from 10 to 30%.

The fifth feature of the present invention relates to durability.Various indications for obtaining durability as desired are describedbelow.

A Young's modulus of the magnetic recording medium is preferably from300 to 2000 kg/mm², and more preferably from 400 to 1500 kg/mm² asmeasured with a tensile tester. With respect to layer (b), a Young'smodulus is preferably from 400 to 5000 kg/mm², and more preferably from500 to 4000 kg/mm² ; a yield stress is preferably from 3 to 20 kg/mm²,and more preferably from 4 to 18 kg/mm² ; and a yield elongation intension is preferably from 0.2 to 8%, and more preferably from 0.5 to5%. These indications of durability are influenced by the ferromagneticpowder, binder, carbon black, inorganic powder, and support used.

A stiffness in flexure (loop stiffness) of the magnetic recording mediumhaving a total thickness of more than 11.5 μm is preferably from 40 to300 mg, that with the total thickness being 10.5±1 μm is preferably from20 to 90 mg, and that with the total thickness being thinner than 9.5 μmis preferably from 10 to 70 mg. This indication of durability is chieflyinfluenced by the support.

An elongation of the magnetic recording medium at which crackinginitiates is preferably not more than 20% as measured at 23° C. and 70%RH.

A Cl/Fe spectrum α of the surface of layer (b) is preferably from 0.3 to0.6, and an N/Fe spectrum β of layer (b) is preferably from 0.03 to0.12, both measured with an X-ray photoelectric spectrophotometer. Theseindications of durability are influenced by the ferromagnetic powder,inorganic powder, and binder used.

The magnetic layer preferably has a glass transition temperature Tg offrom 40 to 120° C. (a maximum peak of loss elastic modulus in themeasurement of dynamic viscoelasticity at 110 Hz), a storage elasticmodulus E' (50° C.) of from 0.8×10¹¹ to 11×10¹¹ dyne/cm², and a losselastic modulus E". (50° C.) of from 0.5×10¹¹ to 8×10¹¹ dyne/cm² asmeasured with a dynamic viscoelastometer. The loss tangent is preferablynot more than 0.2. If the loss tangent is too high, tack troubles tendto develop. These properties are important indications of durability andare influenced by the binder, carbon black, and solvent used.

An adhesive strength between the non-magnetic support and layer (b) ispreferably not less than 10 g as measured by 180° peel test on a 8 mmwide specimen at 23° C. and 70% RH.

A steel ball wear of the surface of layer (b) is preferably from0.7×10⁻⁷ to 5×10⁻⁷ m³ at 23° C. and 70% RH. This is chiefly concernedwith a ferromagnetic powder and gives a direct indication of wearresistance (durability) of the magnetic layer surface.

When five electron micrographs (×50000) taken of the magnetic recordingmedium with SEM are observed with the naked eye, the number of abrasivegrains appearing on the surface of the magnetic laeyr is preferably notless than 0.1 per μm². The number of abrasive grains present on thesurface of the end of the upper magnetic layer is preferably not lessthan 5/μm². These indications of durability are influenced by theabrasive and binder used in the magnetic layer.

A residual solvent in the magnetic recording medium as measured by gaschromatography is preferably not more than 50 mg/m². A residual solventin layer (b) is preferably not more than 20 mg/m², and more preferablynot more than 10 mg/m². The residual solvent in layer (b) is preferablylower than that in layer (a).

A sol fraction (a ratio of THF solvent solids content to weight of layer(b)) of the magnetic recording medium is preferably not more than 15%.This indication of durability is influenced by the ferromagnetic powderand binder used.

When information is recorded on the magnetic recording medium at a shortwavelength (1 MHz) and developed by magnetic development withFerricolloid, the number of continuous black or white lines appearing ina 5 mm wide specimen is preferably not more than 5 as observed under adifferential interference microscope (×10).

A coefficient of friction (μ) of the magnetic recording medium ispreferably from 0.15 to 0.4, and more preferably from 0.2 to 0.35, onits magnetic side and from 0.15 to 0.4, and more preferably from 0.2 to0.35, on its back side.

A contact angle with water of the magnetic recording medium ispreferably from 60° to 130°, and more preferably from 80° to 120°. Acontact angle with methylene iodide is preferably from 10° to 90°, andmore preferably from 20° to 70°. These contact angles are decided by thelubricant or dispersing agent used.

A surface free energy of each of the magnetic layer and the back layeris preferably from 10 to 100 dyne/cm.

A surface resistivity of each of the magnetic layer and the back layeris preferably not more than 1×10⁹ Ω/sq, and more preferably not morethan 1×10⁸ Ω/sq.

In the following description, other materials which can be used in thepresent invention are described. Also, other general aspects of thepresent invention are described below, which are subject to furtherlimitations set forth in the above descriptions of the preferredembodiments depending on which particular embodiment of the invention ispracticed.

The non-magnetic inorganic powder which can be used in the presentinvention includes metals, metal oxides, metal carbonates, metalsulfates, metal nitrides, metal carbides, and metal sulfides. Specificexamples of these inorganic compounds include TiO₂ (rutile or anatase),TiO_(x), cerium oxide, tin oxide, tungsten oxide, ZnO, ZrO₂, SiO₂, Cr₂O₃, α-alumina (α ratio: 90% or more), β-alumina, γ-alumina, α-ironoxide, goethite, corundum, silicon nitride, titanium carbide, magnesiumoxide, boron nitride, molybdenum disulfide, copper oxide, MgCO₃, CaCO₃,BaCO₃, SrCO₃, BaSO₄, silicon carbide, and titanium carbide, either aloneor in combination of two or more thereof. These inorganic powders arenot limited in shape or size. If desired, different inorganic powdersmay be used in combination, or a single powder having a selected sizedistribution can be used to the same effect.

A particle size of the non-magnetic inorganic powder is preferablyaccordant with the particular embodiment selected. In general,particulate, spherical or polygonal particles have a particle sizeusually of from 0.01 to 0.7 μm and preferably not more than 1/4 theshortest recording wavelength λ. Acicular particles have a major axislength usually of from 0.05 to 1.0 μm, and preferably of from 0.05 to0.5 μm, with an acicular ratio usually of from 5 to 20, and preferablyof from 5 to 15. Tabular particles have a plate diameter usually of from0.05 to 1.0 μm, and preferably of from 0.05 to 0.5 μm, with an aspectratio (plate diameter/thickness ratio) usually of from 5 to 20, andpreferably of from 10 to 20.

The inorganic powders have a tapped density usually of from 0.05 to 2g/cc, and preferably of from 0.2 to 1.5 g/cc; a water content usually offrom 0.1 to 5%, and preferably of from 0.2 to 3%; a pH usually of from 2to 11, and preferably of from 4 to 10; a specific surface area usuallyof from 1 to 100 m² /g, preferably of from 5 to 70 m² /g, and morepreferably of from 7 to 50 m² /g; a crystallite size of from 0.01 to 2μm; a DBP absorption usually of from 5 to 100 ml/100 g, preferably offrom 10 to 80 ml/100 g, and more preferably of from 20 to 60 ml/100 g; aspecific gravity usually of from 1 to 12, and preferably of from 3 to 6;a stearic acid (SA) adsorption usually of from 1 to 20 μmol/m², andpreferably of from 2 to 15 μmol/m² ; and a surface roughness factorusually of from 0.8 to 1.5. A heat of wetting in water (25° C.) ispreferably from 200 to 600 erg/cm². Solvents in which the inorganicpowder generates a heat of wetting within this range can be used. Thenumber of water molecules present on the surface of the inorganic powderat 100° to 400° C. is suitably from 1 to 10/100 Å. An isoelectric pointof the inorganic powder in water is preferably between pH 3 and pH 9.

The inorganic powder does not need to be 100% by weight pure and, ifdesired, may be surface-treated with other compounds, e.g., compounds ofAl, Si, Ti, Zr, Sn, Sb, Zn, etc., to form thereon a coat of an oxide ofsuch element according to the purpose sought. In this case, a purity of70% by weight would be enough. An ignition loss of the inorganic powderis preferably not more than 20% by weight.

Specific examples of these inorganic powders include UA 5600 and US 5605(both produced by Showa Denko K. K.); AKP-20, AKP-30, AKP-50, HIT-50,HIT-100, and ZA-G1 (all produced by Sumitomo Chemical Co., Ltd.); G5,G7, and S-1 (all produced by Nippon Chemical Industrial Co., Ltd.;TF-100, TF-120, TF-140, R 516 (all produced by Toda Kogyo K. K.);TTO-51B, TTO-55A, TTO-55B, TTO-55C, TTO-55S, TTO-55D, FT-1000, FT-2000,FTL-100, FTL-200, M-1, S-1, SN-100, R-820, R-830, R-930, R-550, CR-50,CR-80, R-680, and TY-50 (all produced by Ishihara Sangyo Kaisha, Ltd.);ECT-52, STT-4D, STT-30D, STT-30, and STT-65C (all produced by TitanKogyo K. K.); T-1 (produced by Mitsubishi Material Corp.); NS-O, NS-3Y,and NS-8Y (all produced by Nippon Shokubai Kagaku Kogyo Co., Ltd.);MT-100S, MT-100T, MT-150W, MT-500B, MT-600B, and MT-100E (all producedby Teika K. K.); FINEX-25, BF-1, BF-10, BF-20, BF-1L, and BF-10P (allproduced by Sakai Kagaku K. K.); DEFIC-Y and DEFIC-R (both produced byDowa Kogyo K. K.); and Y-LOP and calcined product thereof (produced byTitan Kogyo K. K.).

Of these non-magnetic inorganic powder particularly preferred for use inthe present invention is titanium oxide, and especially titaniumdioxide. Process for producing titanium oxide is described below indetail. Preparation of titanium oxide chiefly includes a sulfuric acidprocess and a chlorine process. According to the former process, a rawore of ilmenite is distilled in sulfuric acid to extract Ti, Fe, etc. inthe form of a sulfate, and iron sulfate is removed by crystallization.After purifying the residual titanium sulfate solution by filtration,hydrolysis is conducted under heating to precipitate hydrous titaniumoxide, which is then filtered and washed to remove impurities. Aparticle size regulator, etc. is added thereto, followed by calcinationat 80 to 1000° C. to obtain crude titanium oxide. The structure type,rutile or anatase, is decided by the kind of a nucleating agent added onhydrolysis. The resulting crude titanium oxide is subjected to finishingtreatments, such as pulverization, classification, surface treatment,and so on. The chlorine process is applied to a naturally-occurringrutile type ore or synthetic rutile. An ore is chlorinated in a reducedstate at a high temperature, whereby Ti is converted to TiCl₄, and Fe toFeCl₂. After cooling, solidified iron oxide is separated from liquidTiCl₄. The resulting crude TiCl₄ is purified by rectification and, afteraddition of a nucleating agent, brought into an instantaneous reactionwith oxygen at 1000° C. or higher to obtain crude titanium oxide, whichis then subjected to the same finishing treatments as described abovefor imparting pigment properties.

In the present invention, layer (a) may further contain carbon black todecrease surface resistivity R_(s) as commonly expected. Carbon blackwhich can be used in the present invention includes furnace black forrubbers, thermal black for rubbers, color black, and acetylene black.Carbon black to be used preferably has a specific surface area usuallyof from 100 to 500 m² /g, and preferably of from 150 to 400 m² /g; a DBPabsorption usually of from 20 to 400 ml/100 g, and preferably of from 30to 200 ml/100 g; an average particle size of from 5 to 80 mμ, preferablyfrom 10 to 50 mμ, and more preferably from 10 to 40 μm; a pH of from 2to 10; a water content of from 0.1 to 10%; and a tapped density of from0.1 to 1 g/cc.

Specific examples of commercially available carbon black which can beused in the present invention are BLACKPEARLS 2000, 1300, 1000, 900,800, 880 and 700 and VULCAN XC-72 (all produced by Cabot Corp.); #3050,#3150, #3250, #3750, #3950, #2400B, #2300, #1000, #970, #950, #900,#850, #650, #40, MA 40, and MA-600 (all produced by Mitsubishi ChemicalIndustries, Ltd.); CONDUCTEX SC (produced by Columbian Co., Ltd.); 8800,8000, 7000, 5750, 5250, 3500, 2100, 2000, 1800, 1500, 1255, and 1250(all produced by Raven); and Ketjenblack EC (produced by Akizo ChemieNederland B.V.). Carbon black having been surface treated with adispersing agent, for example, resin-grated carbon black, or carbonblack having its surface partly graphitized may be employed. Carbonblack may previously be dispersed in a binder before it is added to anon-magnetic coating composition. The above-described carbon blackspecies may be used either individually or in combination thereof.

In choosing a carbon black species for use in the present invention,reference can be made to, for example, Carbon Black Kyokai (ed.) CarbonBlack Binran.

The non-magnetic organic powder which can be used in the presentinvention includes acryl-styrene resin powders, benzoguanamine resinpowders, melamine resin powders, phthalocyanine pigments, polyolefinresin powders, polyester resin powders, polyamide resin powders,polyimide resin powders, and polyfluoroethylene resin powders. Theseresin powders can be prepared by, for example, the processes describedin JP-A-62-18564 and JP-A-60-255827.

These non-magnetic powders are usually used at a weight ratio to binderof from 20 to 0.1 and a volume ratio to binder of from 10 to 0.1.

In general magnetic recording media, it is common to form a subbinglayer on a support. However, such a subbing layer, as having a thicknessof not more than 0.5 μm and being provided for the purpose of improvingadhesive strength between a support and a magnetic layer, etc., entirelydifferent from the lower non-magnetic layer according to the presentinvention. It is, as a matter of course, recommended to form a subbinglayer for improving adhesion between a support and the lower layer.

Ferromagnetic powders which can be used in the upper magnetic layerinclude γ-FeO_(x) (x=1.33 to 1.5), Co-doped γ-FeO_(x) (x=1.33 to 1.5),ferromagnetic alloy fine powders comprising Fe, Ni or Co as a maincomponent (75 atom % or more), barium ferrite, and strontium ferrite.These ferromagnetic powders may optionally contain Al, Si, S, Sc, Ti, V,Cr, Cu, Y, Mo, Rh, Pd, Ag, Sn, Sb, Te, Ba, Ta, W, Re, Au, Hg, Pb, Bi,La, Ce, Pr, Nd, P, Co, Mn, Zn, Ni, Sr, B, for example, in addition tothe prescribed elements. The ferromagnetic powder may previously betreated with a dispersing agent, a lubricant, a surface active agent, anantistatic agent, and the like, as hereinafter described. Specificexamples of suitable ferromagnetic powders are described, e.g., inJP-B-44-14090, JP-B-45-18372, JP-B-47-22062, JP-B-47-22513,JP-B-46-28466, JP-B-46-38755, JP-B-47-4286, JP-B-47-12422,JP-B-47-17284, JP-B-47-18509, JP-B-47-18573, JP-B-39-10307,JP-B-48-39639, and U.S. Pat. Nos. 3,026,215, 3,031,341, 3,100,194,3,242,005, and 3,389,014.

Of the above-mentioned ferromagnetic powders, ferromagnetic alloypowders may contain a small amount of a hydroxide or an oxide. Theferromagnetic alloy fine powders can be prepared by known processes,such as a process comprising reduction with a reducing gas, e.g.,hydrogen in the presence of a complex organic acid salt (mainly anoxalate), a process comprising reducing iron oxide with a reducing gas,e.g., hydrogen to obtain Fe or Fe--Co particles, a process comprisingpyrolysis of a metal carbonyl compound, a process comprising adding toan aqueous solution of a ferromagnetic metal a reducing agent, e.g.,sodium borohydride, a hypophosphite, or hydrazine to conduct reduction,and a process comprising evaporating a metal in a low pressure inert gasto obtain a fine powder. The thus obtained ferromagnetic alloy powdermay further be subjected to a conventional slow oxidation treatment by,for example, a process comprising dipping in an organic solvent followedby drying; a process comprising dipping in an organic solvent, blowingan oxygen-containing gas to form an oxidized film on the surface of thepowder, followed by drying; or a process comprising forming an oxidizedfilm on the powder surface using a mixed gas of oxygen and an inert gasat a controlled mixing ratio in the absence of an organic solvent.

The ferromagnetic powder in the upper magnetic layer has a BET specificsurface area of from 25 to 80 m² /g, and preferably from 35 to 60 m² /g.If the specific surface area is less than 25 m² /g, noise tends to beincreased. If it exceeds 80 m² /g, satisfactory surface properties maynot be obtained. The ferromagnetic powder generally has a crystallitesize of from 100 to 450 Å, and preferably from 100 to 350 Å. Saturationmagnetization σ_(s) of an iron oxide magnetic powder is 50 emu/g ormore, and preferably 70 emu/g or more, and that of the ferromagneticmetal fine powder is preferably 100 emu/g or more, more preferably from110 to 170 emu/g. The ferromagnetic powder preferably has a coerciveforce of 1100 to 2500 Oe and more preferably 1400 to 2000 Oe, and anacicular ratio of 18 or less and more preferably 12 or less.

Percent residual magnetism r₁₅₀₀ of the ferromagnetic powder, which isobserved when a magnetic recording medium is magnetized to saturationand then a magnetic field of 1500 Oe is applied to the oppositedirection, is preferably not more than 1.5%, and more preferably notmore than 1.0%.

The ferromagnetic powder preferably has a water content of from 0.01 to2% by weight. However, the water content of the ferromagnetic powder ispreferably optimized according to the kind of a binder to be used.γ-Iron oxide preferably has a tapped density of 0.5 g/cc or more, andmore preferably 0.8 g/cc or more. In the case of alloy powder, thetapped density is preferably 0.2 to 0.8 g/cc. If it is more than 0.8g/cc the alloy powder is liable to be oxidized during preparation ofmangnetic coatings, resulting in insufficient saturation magnetization(σ_(s)), and if it less than 0.2 g/cc, dispersibility of the powder maybe deteriorated.

In using γ-iron oxide, a ratio of divalent iron to trivalent ironpreferably ranges from 0 to 20 atom %, and more preferably from 5 to 10atom %. A ratio of a cobalt atom to an iron atom is preferably from 0 to15 atom %, and more preferably from 2 to 8 atom %.

It is preferable to optimize the pH of the ferromagnetic powder bycombination with a binder to be used. The pH range of the ferromagneticpowder is from 4 to 12, and preferably from 6 to 10. If desired, theferromagnetic powder may be subjected to a surface treatment with from0.1 to 10% by weight of Al, Si, or P, or an oxide thereof based on theferromagnetic powder. Such a surface treatment reduces an adsorption ofa lubricant, e.g., fatty acids, to 100 mg/m² or less. Existence ofsoluble inorganic ions, e.g., Na, Ca, Fe, Ni, and Sr, observed in someferromagnetic powders cause no adverse influence to characteristics aslong as their concentration is below 500 ppm.

The void of the ferromagnetic powder is preferably as low as possible,preferably not more than 20% by volume, and more preferably not morethan 5% by volume. The shape of the ferromagnetic powder is not limitedas long as the above-defined requirements with respect to particle sizeare satisfied and may be an acicular form, a particulate form, a grainform, or a tabular form. In order to control SFD of the ferromagneticpowder to 0.6 or smaller, it is necessary to narrow the Hc distributionof the powder. This can be achieved by, for example, optimization ofparticle size distribution of goethite, prevention of sintering ofγ-hematite, and making cobalt deposition slower than conventionallyemployed in the preparation of Co-doped iron oxide. With respect to SFD,reference can be made to C. Denis Mee and Eric D. Daniel, Magneticrecording, pub. by MvGraw-Hill, Inc. (1987).

In addition, tabular hexagonal ferromagnetic powders having an axis ofeasy magnetization in the direction perpendicular to the plate can alsobe used in the present invention. Examples of tabular hexagonalferromagnetic powders includ barium ferrite, strontium ferrite, leadferritle, calcium ferrite, cobalt ferrite, and hexagonal Co powder. Morespecifically, the hexagonal tabular ferrite includes barium ferrite andstrontium ferrite of magnetoplumbite type and barium ferrite orstrontium ferrite of magnetoplumbite type locally containing a spinelphase. Particularly preferred are barium ferrite and strontium ferrite.In order to control coercive force, the above-mentioned hexagonalferrite having added thereto Co--Ti, Co--Ti--Zr, Co--Ti--Zn, Ni--Ti--Zn,Ir--Zn, etc. can be used.

In addition, tabular hexagonal ferrites, e.g., barium ferrite andstrontium ferrite, and hexagonal Co powder can also be used as aferromagnetic powder. Barium ferrite to be used has a particle diameterof from 0.001 to 1 μm, a thickness/diameter ratio of from 1/2 to 1/20, aspecific gravity of from 4 to 6 g/cc, and a specific surface area offrom 1 to 60 m² /g.

Carbon black which can be used in the upper magnetic layer, inparticular, includes furnace black for rubbers, thermal black forrubbers, color black, and acetylene black. Carbon black to be usedpreferably has a specific surface area of from 5 to 500 m² /g, a DBP oilabsorption of from 10 to 400 ml/100 g, a particle size of from 5 to 300mμ, a pH of from 2 to 10, a water content of from 0.1 to 10% by weight,and a tapped density of from 0.1 to 1 g/cc. Specific examples ofcommercially available carbon black which can be used in the presentinvention are BLACKPEARLS 2000, 1300, 1000, 900, 800, and 700 and VULCANXC-72 (all produced by Cabot Corp.); #80, #60, #55, #50, and #35 (allproduced by Asahi Carbon); #2400B, #2300, #900, #1000, #30, #40, and #10(all produced by Mitsubishi Chemical Corporation); CONDUCTEX SC andRAVEN 150, 50, 40, and 15 (all produced by Columbian Carbon Co., Ltd. ).Carbon black having been surface treated with a dispersing agent, forexample, resin-grafted carbon black, or carbon black having its surfacepartly graphitized may be employed. Carbon black may previously bedispersed in a binder before it is added to a non-magnetic coatingcomposition. The above-described carbon black species may be used eitherindividually or in combination thereof. Carbon black, if used, ispreferably added in an amount of from 0.1 to 30% by weight based on theferromagnetic powder in the layer.

Carbon black functions to prevent static charge, to reduce coefficientof friction, to screen light, and to improve film strength. Theseperformances of carbon black vary depending on the particular speciesselected. Accordingly, the kind, amount or combination of carbon blackspecies to be used in the lower non-magnetic layer and the uppermagnetic layer can be selected appropriately depending on the purposesought while taking into consideration the above-mentioned variouscharacteristics, e.g., particle size, oil absorption, conductivity, andpH. For example, as noted above, carbon black having high conductivitymay be used in the lower non-magnetic layer for static chargeprevention, and carbon black of large size may be used in the uppermagnetic layer for reduction of coefficient of friction. In choosingcarbon black for use in the present invention, Carbon Black Kyokai(ed.), Carbon Black Binran can be referred to.

Abrasives which are usually used in the upper magnetic layer, inparticular, include known materials usually having a Mohs hardness of 6or more, e.g., α-alumina (α ratio: 90% or more), β-alumina, siliconcarbide, chromium oxide, cerium oxide, α-iron oxide, corundum,artificial diamond, silicon nitride, silicon carbide, titanium carbide,titanium oxide, silicon dioxide, boron nitride, and the like, eitheralone or in a combination of two or more thereof. A complex of theseabrasives (an abrasive having been surface-treated with anotherdifferent abrasive) may be employed. These abrasives may containelements other than the main constituent elements in a proportion of upto 10% by weight. The abrasive preferably has a particle size of from0.01 to 2 μm. If desired, a plurality of abrasives different in particlesize may be used in combination, or a single kind of an abrasive havinga broad size distribution can be used to the same effect. The abrasivepreferably has a tapped density of from 0.3 to 2 g/cc, a water contentof from 0.1 to 5% by weight, a pH of from 2 to 11, and a specificsurface area of from 1 to 30 m² /g. The shape of the abrasive may be anyof an acicular shape, a spherical shape, and a cubic shape. An abrasivepartly having an angular shape is preferred in view of abrasiveperformance.

Specific examples of usable abrasives are AKP-20, KP-30, AKP-50, andHIT-50 (all produced by Sumitomo Chemical Co., Ltd.); G5, G7, and S-1(all produced by Nippon Chemical Industrial Co., Ltd.); and 100Ed and140Ed (both produced by Toda Kogyo K. K.).

The abrasive may previously be dispersed in a binder and then added to amagnetic coating composition. The amount of the abrasive in the uppermagnetic layer preferably ranges from 5 or more particles per 100 μm².

Binders which can be used in the lower non-magnetic layer and the uppermagnetic layer of the present invention include conventionally knownthermoplastic resins, thermosetting resins and reactive resins, andmixtures thereof. The thermoplastic resins to be used have a glasstransition temperature of from 100° to 150° C., a number averagemolecular weight of from 1000 to 200,000, and preferably from 10,000 to100,000, and a degree of polymerization of from about 50 to 1000.Examples of such thermoplastic resins include polymers and copolymerscomprising vinyl chloride, vinyl acetate, vinyl alcohol, maleic acid,acrylic acid, an acrylic ester, vinylidene chloride, acrylonitrile,methacrylic acid, a methacrylic ester, styrene, butadiene, ethylene,vinyl butyral, vinyl acetal, a vinyl ether, and the like; polyurethaneresins, and various rubbery resins. The thermosetting resins andreactive resins include phenol resins, epoxy resins, polyurethane curingresins, urea resins, melamine resins, alkyd resins, acrylic reactiveresins, formaldehyde resins, silicone resins, epoxy-polyamide resins, amixture of a polyester resin and an isocyanate prepolymer, a mixture ofa polyester polyol and a polyisocyanate, and a mixture of a polyurethaneand a polyisocyanate.

For the details of these binder resins, Plastic Handbook published byAsakura Shoten can be referred to. A known electron-curable resin may beused in the magnetic and non-magnetic layers. With respect to specificexamples of the electron-curable resin and the processes for producingthem, reference can be made to JP-A-62-256219. The above-mentionedbinder resins can be used either individually or in a combinationthereof. A preferred combined binder system comprises a polyurethaneresin, at least one of a vinyl chloride resin, a vinyl chloride-vinylacetate resin, a vinyl chloride-vinyl acetate-vinyl alcohol resin, and avinyl chloride-vinyl acetate-maleic anhydride resin and, if desired, apolyisocyanate.

The polyurethane resin may have a conventional structure and includespolyester polyurethane, polyether polyurethane, polyether polyesterpolyurethane, polycarbonate polyurethane, polyester polycarbonatepolyurethane, and polycaprolactone polyurethane.

If desired, all the above-mentioned binders may have bonded thereto oneor more polar groups, e.g., SO₃ M¹, COOM¹, OSO₃ M¹, P═O(OM²)(OM³),--OP═O(OM²)(OM³), --NR₄ X (wherein M¹, M² and M³ each represents ahydrogen atom, Li, Na, K, --NR4 or --NHR₃ ; R represents a hydrogen atomor an alkyl group; and X represents a halogen atom), OH, NR₂, N⁺ R₃(wherein R represents a hydrocarbon group), an epoxy group, SH, and CN,by copolymerization or addition reaction to thereby improvedispersibility and durability. Such a polar group is incorporated in anamount of from 10⁻¹ to 10⁻⁸ mol/g, and preferably from 10⁻² to 10⁻⁶mol/g.

The vinyl chloride copolymers preferably include epoxy-containing vinylchloride copolymers, such as those comprising a vinyl chloride repeatingunit, an epoxy-containing repeating unit and, if desired, a repeatingunit containing a polar group, e.g., --SO₃ M, --OSO₃ M, --COOM, and--PO(OM)₂, wherein M is a hydrogen atom or an alkali metal.Epoxy-containing vinyl chloride copolymers comprising anepoxy-containing repeating unit and a repeating unit containing --SO₃ Naare preferred.

The repeating unit containing a polar group is present in the copolymerin a proportion usually of from 0.01 to 5.0 mol %, and preferably offrom 0.5 to 3.0 mol %. The epoxy-containing repeating unit is present inan amount usually of from 1.0 to 30 mol %, and preferably of from 1 to20 mol %, based on the copolymer or in an amount usually of from 0.01 to0.5 mol, and preferably of from 0.01 to 0.3 mol, per mol of the vinylchloride repeating unit.

If the epoxy-containing repeating unit content is lower than 1 mol %based on the copolymer or lower than 0.01 mol per mol of the vinylchloride repeating unit, there is a tendency that release of hydrogenchloride from the vinyl chloride copolymer cannot be preventedeffectively. If the repeating unit content is more than 30 mol % basedon the copolymer, or more than 0.5 mol per more of the vinyl chloriderepeating unit, hardness of the copolymer is insufficient, and use ofsuch a vinyl chloride resin sometimes causes reduction in runningdurability.

If the content of the repeating unit containing the specific polar groupis less than 0.01 mol %, dispersibility of the ferromagnetic powdertends to be insufficient. If it exceeds 5.0 mol %, the copolymer tendsto show hygroscopicity, resulting in reduction in weather resistance.

The above-describe vinyl chloride copolymer usually has a number averageparticle size of from 15,000 to 60,000.

The vinyl chloride copolymer containing an epoxy group and a specificpolar group, e.g.,, --SO₃ Na, can be prepared by mixing a monomer havinga reactive double bond and epoxy group with a monomer having a reactivedouble bond and a polar group, e.g., --SO₃ Na, at a low temperature andpolymerizing the mixture in the presence of vinyl chloride underpressure at a temperature of 100° C. or lower.

Examples of the monomer having a reactive double bond and an epoxy groupusually include glycidyl (meth)acrylate.

Examples of the monomer having a reactive double bond and a polar groupinclude 2-(meth)acrylamide-2-methylpropanesulfonic acid or a sodium saltthereof, vinylsulfonic acid and its sodium or potassium salt,(meth)acrylic acid-2-ethyl sulfonate and a sodium or potassium saltthereof, maleic acid, maleic anhydride, (meth)acrylic acid, and(meth)acrylic acid-2-phosphate.

The epoxy-containing vinyl chloride copolymer may also be prepared byutilizing a method for introducing a polar group into a polymer, inwhich a polyhydric vinyl chloride copolymer is prepared from, forexample, vinyl chloride and vinyl alcohol, and the resulting copolymeris reacted with a compound having a polar group and a chlorine atom(dehydrochlorination reaction).

Examples of the compound having a polar group and a chlorine atominclude ClCH₂ CH₂ SO₃ M, ClCH₂ CH₂ OSO₃ M, ClCH₂ COOM, and ClCH₂PO(OM)₂, wherein M is as defined above.

In this case, epichlorohydrin is usually used for introduction of anepoxy group.

The vinyl chloride copolymer may further contain other monomers, such asvinyl ethers (e.g., methyl vinyl ether, isobutyl vinyl ether, laurylvinyl ether), α-monoolefins (e.g., ethylene, propylene), (meth)acrylicesters having a functional group (e.g., methyl (meth)acrylate,hydroxyethyl (meth)acrylate), unsaturated nitrites (e.g.,(meth)acrylonitrile), aromatic vinyl compounds (e.g., styrene,α-methylstyrene), and vinyl esters (e.g., vinyl acetate, vinylpropionate)

Specific examples of commercially available binder resins which can beused in the present invention are VAGH, VYHH, VMCH, VAGF, VAGD, VROH,VYES, VYNC, VMCC, XYHL, XYSG, PKHH, PKHJ, PKHC, and PKFE (all producedby Union Carbide Co., Ltd.); MPR-TA, MPR-TA5, MPR-TAL, MPR-TSN, MPR-TMF,MPR-TS, and MPR-TM (all produced by Nisshin Chemical Industry Co.,Ltd.); 1000W, DX80, DX81, DX82, and DX83 (all produced by Denki KagakuCo., Ltd.); MR110, MR100, and 400X110A (all produced by Nippon Zeon Co.,Ltd.); Nippollan N2301, N2302, and N2304 (all produced by NipponPolyurethane Co., Ltd.); Pandex T-5105, T-R3080, and T-5201, VarnokD-400 and D-210-80, and Crysvon 6109 and 7209 (all produced by DainipponInk Co., Ltd.); Vylon UR8200, UR8300, RV530, and RV280 (all produced byToyobo Co., Ltd.); Daipheramine 4020, 5020, 5100, 5300, 9020, 9022, and7020 (all produced by Dainichi Seika Co., Ltd.); MX5004 (produced byMitsubishi Chemical Corporation); Sunprene SP-150 (produced by SanyoKasei Co., Ltd.); and Salan F310 and F210 (both produced by Asahi KaseiKogyo Kabushiki Kaisha).

The binder is used in the upper magnetic layer in an amount of from 5 to50% by weight, and preferably from 10 to 30% by weight, based on theferromagnetic powder. It is preferable to use a combination of 5 to 30%by weight of a vinyl chloride resin, 2 to 20% by weight of apolyurethane resin, and 2 to 20% by weight of a polyisocyanate, based onthe ferromagnetic powder.

The binder is used in the lower non-magnetic layer in a total amount offrom 5 to 50% by weight, and preferably from 10 to 35% by weight, basedon the non-magnetic powder. It is preferable to use a combination of 3to 30% by weight of a vinyl chloride resin, 3 to 30% by weight of apolyurethane resin, and 0 to 20% by weight of a polyisocyanate compound,each based on the non-magnetic powder.

Where an epoxy-containing resin having a molecular weight of more than30,000 is used in an amount of from 3 to 30% by weight based on thenon-magnetic powder, resins other than the epoxy-containing resin can beused in an amount of from 3 to 30% by weight based on the non-magneticpowder. A polyurethane resin can be used in an amount of from 3 to 30%by weight, and a polyisocyanate compound can be used in an amount offrom 0 to 20% by weight, each based on the non-magnetic powder. Theepoxy group content preferably ranges from 4×10⁻⁵ to 16×10⁴ eq/g basedon the total weight of the binder (inclusive of a curing agent).

The polyurethane resin used as a binder preferably has a glasstransition temperature of from -50° to 100° C., an elongation at breakof from 100 to 2000%, a breaking stress of from 0.05 to 10 kg/cm², and ayield point of from 0.05 to 10 kg/cm².

The amount of the binder, the proportion of a vinyl chloride resin, apolyurethane resin, a polyisocyanate or other resins in the totalbinder, the molecular weight of the binder, the polar group content ofthe binder, and various physical properties of the binder may be variedamong the upper magnetic layer and the lower non-magnetic layer asneeded.

The polyisocyanate which can be used in the present invention includesisocyanate compounds, e.g., tolylene diisocyanate, 4,4'-diphenylmethanediisocyanate, hexamethylene diisocyanate, xylylene diisocyanate,naphthylene-1,5-diisocyanate, o-toluidine diisocyanate, isophoronediisocyanate, and triphenylmethane triisocyanate; a reaction productbetween such an isocyanate compound and a polyhydric alcohol; and apolyisocyanate compound obtained by condensation of an isocyanatecompound. These isocyanate compounds are commercially available undertrade names of Coronate L, Coronate HL, Coronate 2030, Coronate 2031,Millionate MR, and Millionate MTL (all produced by Nippon PolyurethaneCo., Ltd.); Takenate D-102, Takenate D-110N, Takenate D-200, andTakenate D-202 (all produced by Takeda Chemical Industries, Ltd.); andDesmodule L, Desmodule IL, Desmodule N, and Desmodule HL (all producedby Sumitomo Bayer Co., Ltd.). These compounds are used in both the lowernon-magnetic layer and the upper magnetic layer either individually orin a combination of two or more thereof, taking advantage of adifference in curing reactivity.

Other additives which can be used in the present invention include thosehaving lubricating effects, antistatic effects, dispersing effects,and/or plasticizing effects. Examples of such additives are molybdenumdisulfide, tungsten disulfide, graphite, boron nitride, graphitefluoride, silicone oil, silicone having a polar group, fattyacid-modified silicone, fluorine-containing silicone,fluorine-containing alcohols, fluorine-containing esters, polyolefins,polyglycols, alkylphosphoric esters and alkali metal salts thereof,alkylsulfuric esters and alkali metal salts thereof, polyphenyl ether,fluorine-containing alkylsulfuric esters and alkali metal salts thereof,monobasic fatty acids having from 10 to 24 carbon atoms (which may havean unsaturated bond or may be branched) and metal salts thereof (e.g.,Li, Na, K, and Cu salts), mono-, di-, tri-, tetra-, penta- or hexahydricalcohols having from 12 to 22 carbon atoms (which may have anunsaturated bond or may be branched), alkoxy alcohols having from 12 to22 carbon atoms, mono-, di- or tri-fatty acid esters of a monobasicfatty acid having from 10 to 24 carbon atoms (which may have anunsaturated bond or may be branched) and a mono- to hexahydric alcoholhaving from 2 to 12 carbon atoms (which may have an unsaturated bond ormay be branched), fatty acid esters of a monoalkyl ether of an alkyleneoxide polymer, fatty acid amides having from 8 to 22 carbon atoms, andaliphatic amines having from 8 to 22 carbon atoms. Specific examples ofthese fatty acid ester additives are lauric acid, myristic acid,palmitic acid, stearic acid, behenic acid, butyl stearate, oleic acid,linoleic acid, linolenic acid, elaidic acid, octyl stearate, amylstearate, isooctyl stearate, octyl myristate, butoxyethyl stearate,anhydrosorbitan monostearate, anhydrosorbitan distearate,anhydrosorbitan tristearate, oleyl alcohol, and lauryl alcohol.

Surface active agents which can also be used include nonionic surfaceactive agents, such as alkylene oxides, glycerin derivatives, glycidolderivatives, and alkylphenol ethylene oxide adducts; cationic surfaceactive agents, such as cyclic amines, ester amides, quaternary ammoniumsalts, hydantoin derivatives, heterocyclic compounds, and phosphonium orsulfonium compounds; anionic surface active agents containing an acidradical, such as carboxylic acids, sulfonic acids, phosphoric acids,sulfuric esters, and phosphoric esters; and amphoteric surface activeagents, such as amino acids, aminosulfonic acids, sulfuric or phosphoricesters of aminoalcohols, and alkyl betaines. The details of thesesurface active agents are described in Kaimen Kasseizai Binran publishedby Sangyo Tosho K. K.

These lubricants, antistatic agents, surface active agents, and thelike, do not need to be 100% by weight pure and may contain impurities,such as isomers, unreacted matters, by-products, decomposition products,and the like, in a proportion preferably below 30% by weight, and morepreferably below 10% by weight.

The kind and amount of the additives, such as a lubricant and a surfaceactive agent, are appropriately selected for each of the lowernon-magnetic layer and the upper magnetic layer. For example, fattyacids are used with their melting points being varied between the twolayers so as to inhibit bleeding; esters are used with their boilingpoints or polarity being varied between the two layers so as to inhibitbleeding; the amount of the surface active agent is adjusted to improvecoating stability; and the amount of the lubricant in the lowernon-magnetic layer is increased to improve lubricating effects.

All or part of the additives may be added in any stage of thepreparation of the coating compositions. For example, the additives maybe mixed with a ferromagnetic powder before kneading; be added duringkneading of a ferromagnetic powder, a binder, and a solvent; be addedduring or after dispersing; or be added immediately before coating.

Typical examples of useful commercially available lubricants includeNAA-102, NAA-415, NAA-312, NAA-160, NAA-180, NAA-174, NAA-175, NAA-222,NAA-34, NAA-35, NAA-171, NAA-122, NAA-142, NAA-160, NAA-173K, hardenedcastor oil fatty acids, NAA-42, NAA-44, Cation SA, Cation MA, Cation AB,Cation BB, Nymeen L-210, Nymeen L-202, Nymeen S-202, Nonion E-208,Nonion E-208, Nonion P-208, Nonion S-207, Nonion K-204, Nonion NS-202,Nonion NS-210, Nonion HS-206, Nonion L-2, Nonion S-2, Nonion S-4, NonionO-2, Nonion LP-20R, Nonion PP-40R, Nonion SP-60R, Nonion OP-80R, NonionOP-85R, Nonion LT-221, Nonion ST-221, Nonion TO-221, Monoguri MB, NonionDS-60, Anon BF, Anon LG, butyl stearate, butyl laurate, and erucic acid(all produced by Nippon Oils & Fats Co., Ltd.); oleic acid (produced byKanto Kagaku K. K.); FAL-205 and FAL-123 (both produced by TakemotoYushi K. K.); Enujerubu LO, Enujerubu IPM, and Sansosyzer E4030 (allproduced by New Japan Chemical Co., Ltd.); TA-3, KF-96, KF-96L, KF-96H,KF 410, KF 420, KF 965, KF 54, KF 50, KF 56, KF-907, KF-851, X-22-819,X-22-822, KF-905, KF-700, KF-393, KF-857, KF-860, KF-865, X-22-980,KF-101, KF-102, KF-103, X-22-3710, X-22-3715, KF-910, and KF-3935 (allproduced by Shin-Etsu Chemical Industry Co., Ltd.); Armide P, Armide C,and Armoslip CP (all produced by Lion Ahmer); Duomin TDO (produced byLion Fat & Oil Co., Ltd.); BA-41G (produced by Nisshin Seiyu K. K.);Profan 2012E, Newpole PE 61, Ionet MS-400, lonet MO-200, Ionet DL-200,Ionet DS-300, Ionet DS-1000, and Ionet DO-200 (all produced by SanyoChemical Industries Co., Ltd.).

Organic solvents which can be used in the present invention includeketones, e.g., acetone, methyl ethyl ketone, methyl isobutyl ketone,diisobutyl ketone, cyclohexanone, isophorone, and tetrahydrofuran;alcohols, e.g., methanol, ethanol, propanol, butanol, isobutyl alcohol,isopropyl alcohol, and methylcyclohexanol; esters, e.g., methyl acetate,butyl acetate, isobutyl acetate, isopropyl acetate, ethyl lactate, andglycol acetate; glycol ethers, e.g., glycol dimethyl ether, glycolmonoethyl ether, and dioxane; aromatic hydrocarbons, e.g., benzene,toluene, xylene, cresol, and chlorobenzene; chlorinated hydrocarbons,e.g., methylene chloride, ethylene chloride, carbon tetrachloride,chloroform, ethylene chlorohydrin, and dichlorobenzene;N,N-dimethylformamide, and hexane; either individually or incombinations thereof at an arbitrary mixing ratio. These organicsolvents do not need to be 100% by weight pure and may containimpurities, such as isomers, unreacted matters, by-products,decomposition products, oxides, water content, etc. in a proportionpreferably below 30% by weight, and more preferably below 10% by weight.If desired, the kind and amount of the organic solvents to be used maybe varied between the upper and lower layers according to the purposesought. For example, a highly volatile solvent may be used in the lowernon-magnetic layer to improve surface properties; a solvent having highsurface tension (e.g., cyclohexanone and dioxane) may be used in theupper magnetic layer to improve coating stability; or a solvent having ahigh solubility parameter may be used in the upper magnetic layer toimprove packing.

The non-magnetic support has a thickness of from 1 to 100 μm, andpreferably from 4 to 80 μm. The lower non-magnetic layer has a thicknessof from 0.5 to 10 μm, and preferably from 1 to 5 μm. The upper magneticlayer has a thickness of not more than 1.0 μm, preferably from 0.05 to1.0 μm, more preferably from 0.05 to 0.6 μm, and most preferably from0.05 to 0.3 μm. The upper magnetic layer of the present invention may beconstituted of two or more layers. In the case, the total thickness ofthe layers should not be more than 1.0 μm. The total thickness of theupper magnetic layer and the lower non-magnetic layer is from 1/100 to 2times the thickness of the non-magnetic support. For the purpose ofimproving adhesion between the non-magnetic support and the lowernon-magnetic layer, there may be provided a subbing layer having athickness of from 0.01 to 2 μm, and preferably from 0.05 to 0.5 μm. Aback coat may be provided on the non-magnetic support on the sideopposite to the magnetic layer to a thickness of from 0.1 to 2 μm, andpreferably from 0.3 to 1.0 μm. The subbing layer and the back coat layercan be formulated from conventional compositions.

The non-magnetic support includes known films of polyesters, e.g.,polyethylene terephthalate and polyethylene naphthalate, polyolefins,cellulose triacetate, polycarbonate, polyamide, polyimide,polyamide-imide, polysulfone, aramide, and aromatic polyamide. Thesupport may be subjected to a pretreatment, such as corona dischargetreatment, plasma treatment, treatment for easy adhesion, heattreatment, and dusting.

In order to accomplish the objects of the present invention, thenon-magnetic support should have a centerline average roughness (Ra) ofnot more than 0.03 μm, preferably not more than 0.02 μm, and morepreferably not more than 0.01 μm. Besides having a small Ra, thenon-magnetic support is preferably free from coarse projections of 1 μmor greater.

The surface roughness is arbitrarily controllable by adjusting the sizeand amount of fillers to be added. Fillers include an oxide or carbonateof Ca, Si, or Ti and organic fine powders, e.g., acrylic resin powders.

The non-magnetic support preferably has an F-5 value (a modulus ofelasticity at 5% elongation) ranging from 5 to 50 kg/mm² in itslongitudinal direction (running direction) and from 3 to 30 kg/mm² inits width direction. The F-5 value in the longitudinal direction isgenerally higher than that in the width direction, but this does notapply when it is required to particularly increase the strength in thewidth direction.

A percent thermal shrinkage of the non-magnetic support in bothlongitudinal and width directions is preferably not more than 3%, andmore preferably not more than 1.5%, at 100° C.×30 minutes and not morethan 1%, and more preferably not more than 0.5%, at 80° C.×30 minutes.The non-magnetic support preferably has a breaking strength of from 5 to100 kg/mm² in both longitudinal and width directions and a modulus ofelasticity of from 100 to 2000 kg/mm².

A magnetic coating composition for the magnetic recording medium of thepresent invention is prepared by a process essentially comprising akneading step and a dispersing step. A mixing step may be insertedbefore and/or after each of the kneading step and the dispersing step.Each step may be conducted in two or more separate stages. All thematerials to be used in the present invention, e.g., a ferromagneticpowder, a binder, carbon black, an abrasive, an antistatic agent, alubricant, and a solvent, may be added in the beginning of or during anystep. Each material may be added in two or more separate portions. Forexample, a separate portion of polyurethane may be added to each of thekneading step, the dispersing step, and a mixing step for viscosityadjustment after dispersing.

While conventional techniques for producing magnetic recording media canbe made use of in the present invention, it is preferable to use akneading apparatus having a high kneading force, such as a continuouskneader and a pressure kneader, in the kneading step for obtaining ahigh Br. In using a continuous kneader or a pressure kneader, aferromagnetic powder, a part (preferably at least 30%) or the wholeamount of a binder, and 15 to 500 parts by weight of a solvent per 100parts by weight of the ferromagnetic powder are kneaded. For the detailsof the kneading step, reference can be made in JP-A-1-106338 andJP-A-64-79274.

The magnetic recording medium according to the present invention can beproduced with higher efficiency by utilizing a simultaneous coatingsystem as disclosed in JP-A-62-212933. In particular, the followingcoating systems are recommended.

1. Layer (a) (lower non-magnetic layer) is coated with a coating devicegenerally employed for magnetic coatings, e.g., a gravure coater, a rollcoater, a blade coater, and an extrusion coater. While the thus coatedlayer is wet, layer (b) (upper magnetic layer) is coated thereon bymeans of an extrusion coater disclosed in JP-B-1-46186, JP-A-60-238179,and JP-A-2-265672.

2. Layers (a) and (b) are coated substantially simultaneously by meansof a single coating head having two slits as disclosed in JP-A-63-88080,JP-A-2-17921, and JP-A-2-265672.

3. Layers (a) and (b) are coated substantially simultaneously by meansof an extrusion coater equipped with a back-up roll as disclosed inJP-A-2-174965.

In order to prevent agglomeration of the ferromagnetic powder whichleads to reductions in electromagnetic characteristics, it isrecommended to add a shear to the coating composition inside the coatinghead.

In the present invention, the coating compositions for layers (a) and(b) are coated on a non-magnetic support according to a wet-on-wetcoating system in which coating of one layer is immediately followed bycoating of another layer while the first layer is in a wet state(successive coating) or a plurality of layers are simultaneously coatedby extrusion coating (simultaneous coating). For the details of thewet-on-wet coating system, JP-A-61-139929 can be referred to. The "wetstate" is the state that a coated composition is odhered to hands at atouch and it may be the state that the coated composition contains morethan 10% that of a solvent added in the coating composition beforecoating.

An example of the above-described successive coating system isillustrated in FIG. 2. Running flexible support 1 made of, e.g.,polyethylene terephthalate is pre-coated with coating composition 2 bymeans of coating apparatus 3. Immediately thereafter, the coated surfaceis smoothed by means of smoothing roll 4. While the precoat is wet,coating composition 5 is coated thereon by means of extrusion coater 6.

An example of the above-described simultaneous extrusion coating systemis illustrated in FIG. 3. Coating compositions 2, 5 for layers (a) and(b), respectively, are simultaneously coated on flexible support 1 bymeans of extrusion coater 8.

After layers (a) and (b) are coated, the layers are subjected tomagnetic orientation, drying, and smoothing to obtain a magneticrecording medium.

Powerful magnetic orientation is required for the production of themagnetic recording medium of the present invention. A combination of asolenoid of 1000 G or higher and cobalt magnet of 2000 G or higher ispreferably used for magnetic field application. It is preferable toconduct moderate drying before magnetic orientation so that theorientation after drying can be optimized.

For calendering, heat-resistant plastic rolls made of, e.g., epoxyresins, polyimide, polyamide, or polyimide-amide, are employed.Smoothing may also be carried out by using metallic rolls. Thecalendering temperature is preferably 70° C. or higher, and morepreferably 80° C. or higher. The linear pressure is preferably 200 kg/cmor more, and more preferably 300 kg/cm or more, and the linear velocityranges from 20 to 700 m/min. After the calendering treatment is carriedout, the magnetic recording medium may be subjected to a heat treatmentat a temperature of 40 to 80° C. to accelerate curing of the magneticlayer, the non-magnetic layer and a back layer if any.

A coefficient of fraction of both sides of the magnetic recording mediumof the present invention against a pole of SUS 420J is preferably notmore than 0.5, and more preferably not more than 0.3. A surfaceresistivity of layer (b) preferably ranges from 10⁴ to 10¹¹ Ω/sq. Asurface resistivity of layer (a), if coated alone, preferably rangesfrom 10⁴ to 10⁸ Ω/sq. A back layer preferably has a surface resistanceof from 10³ to 10⁹.

Both layers (a) and (b) preferably has a void of not more than 30% byvolume, and more preferably not more than 20% by volume. The smaller thevoid, the better for achieving high output. For some purposes, however,it is better to control the void above a certain value. For instance, tohave a large void is often favorable for running durability in the caseof magnetic recording media for data recording which are required towithstand repeated use. It is easy for one skilled in the art to controlthe void within a preferred range for the particular purpose.

Layer (b) preferably has a modulus of elasticity at 0.5% elongation offrom 100 to 2000 kg/mm² in both longitudinal and width directions and abreaking strength of from 1 to 30 kg/cm². The magnetic recording mediumpreferably has a modulus of elasticity of from 100 to 1500 kg/mm² inboth longitudinal and width directions, a residual elongation of notmore than 0.5%, and a percent thermal shrinkage of not more than 1%,more preferably not more than 0.5%, and most preferably not more than0.1%, at any temperature below 100° C.

The amount of a residual solvent in layer (b) is preferably not morethan 100 mg/m², and more preferably not more than 10 mg/m². The residualsolvent in layer (b) is preferably less than that in layer (a).

As magnetic characteristics, the magnetic recording medium of thepresent invention has a squareness ratio (Rs) in the running directionof not less than 0.70, preferably not less than 0.80, and morepreferably not less than 0.90, as measured in a magnetic field of 5 KOe.An Rs in the two directions perpendicular to the running direction ispreferably 80% or less that of the running direction. An SFD of themagnetic layer is preferably not more than 0.6.

As could be expected, layers (a) and (b) may have different physicalproperties according to purposes sought for each layer. For example, themodulus of elasticity of layer (b) may be increased to improve runningdurability and, at the same time, the modulus of elasticity of layer (a)is made lower than that of layer (b) to improve contact with a recordinghead.

While the magnetic recording medium according to the present inventionbasically comprises a non-magnetic support having thereon layer (a) andlayer (b). If desired, layer (b) may be composed of two or more magneticlayers according to the layer structure of conventional multi-layeredmagnetic layer. For example, the uppermost magnetic layer may have ahigher coercive force than the lower magnetic layer and contain aferromagnetic powder having a smaller average major axis length orcrystallite size. Also, layer (a) may be composed of two or morenon-magnetic layers.

The present invention is now illustrated in greater detail withreference to Examples, but it should be understood that the presentinvention is not construed as being limited thereto. All the parts,percents, and ratios are given by weight unless otherwise specified.

EXAMPLE 1 AND COMPARATIVE EXAMPLE 1

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Particulate TiO.sub.2 (average particle                                                                100    parts                                         size: 0.09 μm)                                                             Carbon black (average particle                                                                         5      parts                                         size: 20 mμ)                                                               Vinyl chloride copolymer (contain-                                                                     8      parts                                         ing -SO.sub.3 Na and epoxy group; molecular                                   weight: 50000)                                                                Polyurethane resin (containing                                                                         5      parts                                         -SO.sub.3 Na; molecular weight: 45000)                                        α-Alumina (average particle size:                                                                5      parts                                         0.2 μm)                                                                    Cyclohexanone            150    parts                                         Methyl ethyl ketone      50     parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 4hours, and 5 parts of polyisocyanate ("Coronate L" produced by NipponPolyurethane Co., Ltd.), 1 part of stearic acid, and 1 part of butylstearate were added thereto to prepare a coating composition for layer(a).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: Fe/Ni/Co alloy                                                                   100    parts                                         (93/3/3; others: Zn, Cr, etc.; Hc:                                            1600 Oe; saturation magnetization σ.sub.s :                             135 emu/g; major axis length: 0.18 μm;                                     acicular ratio: 9)                                                            Vinyl chloride copolymer (containing                                                                   10     parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na,                                                           5      parts                                         molecular weight: 45000)                                                      α-Alumina (average particle size: 0.2 μm)                                                     5      parts                                         Cyclohexanone            150    parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 6hours. To the dispersion were added 5 parts of polyisocyanate (CoronateL), 5 parts of stearic acid, and 10 parts of butyl stearate to prepare acoating composition for layer (b).

A 9.8 μm thick polyethylene terephthalate film was coated with theabove-prepared coating compositions for layers (a) and (b) to a drythickness of 3.0 μm and 0.3 μm, respectively, by simultaneous wet-on-wetcoating using two doctor blades set at different gaps. The magneticpowder was orientated with a permanent magnetic, and the coated layerswere dried. On the opposite side of the support was provided a backlayer containing carbon black. The both sides of the coated film werecalendered. The resulting film was cut to a width of 3.81 mm to preparedigital audio tape (DAT). The resulting sample was designated 1-1.

Samples 1-2 to 1-9 and Comparative Samples 1'-1 to 1'-6 were prepared inthe same manner as for Sample 1-1, except for making alterationsaccording to Table 1 (alterations to layer (b)) and Table 2 (alterationsto layer (a)) shown below.

An average dry thickness (d) of layer (b), a standard deviation (a) ofd, and an average thickness variation (.sup.Δ d) were determinedaccording to the methods described above. The results obtained are shownin Tables 1 and 2.

Electrophotographic characteristics and coating defects (pinholes) ofeach of the resulting tape samples were evaluated according to thefollowing test methods., and the results obtained are shown in Table 3below. In carrying out the electrophotographic tests, a DAT deck"DTC-1000" produced by Sony Corporation was used.

Reproduction Output

Signals of a single frequency 4.7 MHz were recorded, and the reproducedsignals were put into a spectrum analyzer, "HP-3585A". The peak of thesignals was read out.

C/N Ratio

A noise spectrum was prepared at the measurement of reproduction outputby means of a spectral analyzer "HP-3585A". A C/N ratio was obtainedfrom a ratio of the reproduction output to the noise level apart fromthe central recording frequency (4.7 MHz) by 0.1 MHz.

Block Error Rate (BER)

A block error rate is a number of error flags per 10000 tracks. ##EQU2##

DAT has a signal processing system of coding analog signals into digitalsignals. One signal comprises 8 bits, and one block comprises 32signals×8 pits=256 bits. Accordingly, one track is constituted by 128blocks.

Signals of two tracks, i.e., 128×2 blocks, were put in memory andshuffled, and the errors were detected. A counter "HP5328A" produced byHewlett Pockard Co. was connected to a standard personal computer.

Dropout

Signals of a single frequency 4.7 MHz were input, and dropouts 0.5 μsecin length were counted with a dropout counter on a threshold level -10dB.

Pinholes

A magnetic layer of a sample before formation of a back layer wasobserved with transmitted white light with naked eye to count the numberof pinholes per 100 m².

                                      TABLE 1                                     __________________________________________________________________________    Layer (b)                                                                                                   Major                                                              Ferromagnetic Powder                                                                     Axis                                                                              Plate                                       Sample                                                                            d     Δd                                                                            σ                                                                          Ra         Length                                                                            Diameter                                    No. (μm)                                                                          d/ (μm)                                                                          Δd/d                                                                       (μm)                                                                          (nm)                                                                             /Ra Kind                                                                              (μm)                                                                           (μm)                                     __________________________________________________________________________    1-1 0.70                                                                             0.45                                                                             0.10                                                                             0.33                                                                             0.11                                                                             4.8                                                                              139.58                                                                            Fe--Ni                                                                            0.18                                            1-2 0.70                                                                             1.04                                                                             0.15                                                                             0.21                                                                             0.07                                                                             6.0                                                                              111.67                                                                            Fe--Ni                                                                            0.18                                            1-3 1.00                                                                             1.79                                                                             0.18                                                                             0.15                                                                             0.08                                                                             5.5                                                                              121.82                                                                            Fe--Ni                                                                            0.18                                            1-4 1.00                                                                             1.79                                                                             0.18                                                                             0.15                                                                             0.06                                                                             8.0                                                                              83.75                                                                             Fe--Ni                                                                            0.18                                            1-5 0.65                                                                             0.97                                                                             0.25                                                                             0.38                                                                             0.04                                                                             7.0                                                                              95.71                                                                             Fe--Ni                                                                            0.18                                            1-6 0.65                                                                             0.97                                                                             0.07                                                                             0.11                                                                             0.02                                                                             4.5                                                                              148.99                                                                            Fe--Ni                                                                            0.18                                            1-7 0.50                                                                             0.75                                                                             0.22                                                                             0.44                                                                             0.07                                                                             6.7                                                                              100.00                                                                            Fe--Ni                                                                            0.18                                            1-8 0.70                                                                             1.04                                                                             0.11                                                                             0.16                                                                             0.06                                                                             5.5                                                                              121.82                                                                            Ba      0.05                                                                  ferrite                                             1-9 1.00                                                                             2.24                                                                             0.12                                                                             0.08                                                                             0.10                                                                             4.2                                                                              159.52                                                                            Fe--Ni                                                                            0.18                                            1'-1                                                                              3.30                                                                             4.93                                                                             -- -- -- 4.0                                                                              167.50                                                                            Fe--Ni                                                                            0.18                                            1'-2                                                                              0.50                                                                             0.75                                                                             0.20                                                                             0.40                                                                             0.25                                                                             16.0                                                                             41.88                                                                             Fe--Ni                                                                            0.18                                            1'-3                                                                              0.60                                                                             0.90                                                                             0.35                                                                             0.58                                                                             0.25                                                                             5.0                                                                              134.00                                                                            Fe--Ni                                                                            0.18                                            1'-4                                                                              0.60                                                                             0.90                                                                             0.50                                                                             0.83                                                                             0.26                                                                             18.0                                                                             37.22                                                                             Fe--Ni                                                                            0.18                                            1'-5                                                                              0.15                                                                             0.22                                                                             0.05                                                                             0.33                                                                             0.21                                                                             4.6                                                                              145.65                                                                            Fe--Ni                                                                            0.18                                            1'-6                                                                              0.70                                                                             1.04                                                                             0.22                                                                             0.31                                                                             0.01                                                                             10.0                                                                             67.00                                                                             Fe--Ni                                                                            0.18                                            __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________    Layer (a)                                                                     Non-Magnetic Powder                                                                             Average Major  Plate                                            Thick-        Particle                                                                          Average                                                                           Axis                                                                              Aci-                                                                             Dia-   Coat-                                 Sample                                                                            ness          Size                                                                              Particle                                                                          Length                                                                            cular                                                                            meter                                                                            Aspect                                                                            ing                                   No. (μm)                                                                           Kind Shape                                                                              (μm)                                                                           Size/                                                                             (μm)                                                                           Ratio                                                                            (μm)                                                                          Ratio                                                                             System                                __________________________________________________________________________    1-1 3.00                                                                              TiO.sub.2                                                                          particulate                                                                        0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1-2 2.50                                                                              "    "    0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1-3 2.10                                                                              "    "    0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1-4 0.80                                                                              "    "    0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1-5 2.50                                                                              alumina                                                                            "    0.15                                                                              0.22              simultaneous                                                                  wet-on-wet                            1-6 2.50                                                                              α ferrite                                                                    acicular     0.60                                                                              8.00      simultaneous                                                                  wet-on-wet                            1-7 2.50                                                                              BN   tabular             0.70                                                                             9.00                                                                              simultaneous                                                                  wet-on-wet                            1-8 2.50                                                                              TiO.sub.2                                                                          particulate                                                                        0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1-9 2.00                                                                              "    "    0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1'-1                                                                              --  --   --                         single layer                          1'-2                                                                              0.40                                                                              TiO.sub.2                                                                          particulate                                                                        0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1'-3                                                                              2.50                                                                              red oxide                                                                          "    0.25                                                                              0.37              simultaneous                                                                  wet-on-wet                            1'-4                                                                              2.50                                                                              α ferrite                                                                    acicular     1.50                                                                              18.0      simultaneous                                                                  wet-on-wet                            1'-5                                                                              3.10                                                                              TiO.sub.2                                                                          particulate                                                                        0.09                                                                              0.13              simultaneous                                                                  wet-on-wet                            1'-6                                                                              2.50                                                                              "    "    0.09                                                                              0.13              successive                                                                    wet-on-dry*                           __________________________________________________________________________     Note: *Layer (b) was coated after layer (a) was coated and dried.        

                  TABLE 3                                                         ______________________________________                                        Electromagnetic Characteristics                                                       Repro-                                                                        duction  C/N                                                          Sample  Output.sup.1)                                                                          Ratio.sup.2)          Pinholes                               No.     (dB)     (dB)     BER.sup.3)                                                                           DO    (/100 m.sup.2)                         ______________________________________                                        1-1     4.50     3.50     7      60    0.00                                   1-2     5.00     5.50     4      70    0.00                                   1-3     3.90     4.80     6      85    0.00                                   1-4     3.00     2.80     1      50    0.00                                   1-5     4.00     2.60     3      90    0.00                                   1-6     5.20     6.30     2      80    0.00                                   1-7     4.70     5.20     6      30    0.00                                   1-8     3.80     4.50     9      95    0.00                                   1-9     1.50     1.80     100    120   0.00                                   1'-1    0.00     0.00     200    150   0.00                                   1'-2    -2.0     -1.50    9000   220   5.00                                   1'-3    1.00     -0.50    2000   65    0.00                                   1'-4    -2.3     -0.36    30000  180   0.00                                   1'-5    0.80     0.50     800    520   3.00                                   1'-6    2.50     2.80     3000   1200  2000                                                                          or more                                ______________________________________                                         Note:                                                                         .sup.1) : The result of Sample 11 was taken as a standard (0.00 dB).          .sup.2) : The result of Sample 11 was taken as a standard (0.00 dB).          .sup.3) : ×10.sup.-6 /m.sup.2                                      

The shortest recording medium λ of the above prepared DAT tapes is 0.67μm. This being applied to λ/4≦d≦3λ, a thickness of layer (b) is in arange of from 0.17 to 2.01 μm. Accordingly, the thickness of layer (b)according to the present invention ranges from 0.17 to 1.0 μm. .sup.Δd≦d/2 leads to .sup.Δ d/d≦0.5.

Samples 1-1 to 1-3 have different thickness ratios of layer (a) to layer(b) with the thickness of layer (b) being fixed. The thickness of layer(b) in Sample 1-1 is nearly the lower limit of the above range; that inSample 1-2 is in the middle of the above range; and that in Sample 1-3is nearly the upper limit of the above range. In Sample 1-4, thethickness of layer (a) is reduced near to the lower limit. Samples 1-5through 1-7 are different in kind of the non-magnetic powder in layer(a), proving that the kind of non-magnetic powders has an influence onRa within the scope of the present invention. Sample 1-8 is an exampleof using barium ferrite as a ferromagnetic powder. Sample 1-9 in whichthe thickness of layer (b) is near to the upper limit showsdeterioration in characteristics as compared with other systemsaccording to the present invention.

Comparative Sample 1'-1 has only layer (b) on the support. ComparativeSample 1'-2 having layer (a) thickness reduced is inferior in coatingproperties and Ra. Comparative Sample 1'-3 using a coarse red oxidepowder (average particle size: 0.25 μm, greater than λ/4=0.1675) has alarge .sup.Δ d and is therefore inferior in C/N ratio. ComparativeSample 1'-4 using an acicular red oxide powder having a long major axishas a large .sup.Δ d and is therefore inferior in Ra. Comparative Sample1'-5 in which the thickness of layer (b) is lower than the lower limitis inferior in various characteristics. Comparative Sample 1'-6 is anexample prepared by coating layer (b) after drying layer (a) and suffersfrom development of many pinholes.

Each of Samples 1-1 to 1-8 satisfying the conditions of d, .sup.Δ d, σ,and Ra is superior to any comparative samples in reproduction output,C/N ratio, BER, dropout, and coating properties.

It has now been proved that coating defects appearing in mere reductionin thickness of a magnetic layer can be prevented by simultaneouswet-on-wet coating of a magnetic layer whose thickness is reduced not toexceed three times the shortest recording wavelength. Further, it isseen that optimization of the magnetic layer thickness in relation tothe shortest recording wavelength according the particular recordingsystem employed brings about an improvement in reproduction output.Furthermore, control of thickness variation of the magnetic layer havinga reduced thickness and smoothing the surface of the magnetic layerbring about an improvement in C/N ratio.

Thus, the present invention provides a coated type magnetic recordingmedium exhibiting excellent electromagnetic characteristics comparableto metallic thin film type magnetic recording media while eliminatingproduction problems and inferior reliability on preservationaccompanying the conventional metallic thin film type magnetic recordingmedia.

EXAMPLE 2 AND COMPARATIVE EXAMPLE 2

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Rutile TiO.sub.2 (average particle size:                                                                90     parts                                        0.035 μm; TiO.sub.2 content: ≧90%; surface                          treating agent: Al.sub.2 O.sub.3 ; BET specific                               surface area: 35 to 45 m.sup.2 /g; DBP                                        absorption: 27 to 38 g/100 g; pH:                                             6.5 to 8)                                                                     Carbon black (average particle size:                                                                    10     parts                                        16 mμ; DBP absorption: 80 ml/100 g;                                        pH: 8.0; BET specific surface area:                                           250 m.sup.2 /g; volatile content: 1.5%)                                       Vinyl chloride-vinyl acetate-vinyl                                                                      12     parts                                        alcohol copolymer (86:13:1; -N(CH.sub.3).sub.3.sup.+ Cl.sup.-                 content: 5 x 10.sup.-6 eq/g; polymerization                                   degree: 400)                                                                  Polyester polyurethane resin (neopentyl-                                                                5      parts                                        glycol/caprolactone polyol/diphenyl-                                          methane-4,4'-diisocyanate (MDI) = 0.9/2.6/1;                                  -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               Butyl stearate            1      part                                         Stearic acid              1      part                                         Methyl ethyl ketone       200    parts                                        Cyclohexanone             80     parts                                        ______________________________________                                    

The above components were kneaded in a continuous kneader and dispersedin a sand mill. To the dispersion were added 1 part of polyisocyanateand 40 parts of butyl acetate, followed by filtration through a filterhaving an average pore size of 1 μm to prepare a coating composition forlayer (a).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: Fe/Zn/Ni alloy                                                                 100      parts                                         (92/4/4; Hc: 1600 Oe; BET specific                                            surface area: 60 m.sup.2 /g; crystallite                                      size: 195 Å; average major axis                                           length: 0.20 μm; acicular ratio: 10;                                       σ.sub.s : 130 emu/g)                                                    Vinyl chloride copolymer (--SO.sub.3 Na content:                                                     12       parts                                         1 × 10.sup.-4 eq/g; polymerization degree: 300)                         Polyester polyurethane resin (neopentyl-                                                             3        parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   --SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                              α-Alumina (average particle size: 0.2 μm)                                                   2        parts                                         Carbon black (average particle size:                                                                 8        parts                                         0.10 μm)                                                                   Butyl stearate         1        part                                          Stearic acid           2        parts                                         Methyl ethyl ketone    200      parts                                         ______________________________________                                    

The above components were kneaded in a continuous kneader and dispersedin a sand mill. To the dispersion were added 3 parts of polyisocyanateand 40 parts of butyl acetate, followed by filtration through a filterhaving an average pore size of 1 μm to prepare a coating composition forlayer (b).

The coating composition for layer (a) was coated on a 7 μm thickpolyethylene naphthalate film support having a centerline averagesurface roughness of 0.01 μm to a dry thickness of 3 μm. Immediatelythereafter while layer (a) was wet, the coating composition for layer(b) was coated thereon to a dry thickness of 0.5 μm (successivewet-on-wet coating). While layers (a) and (b) were wet, theferromagnetic powder in layer (b) was orientated by applying a magneticfield using a cobalt magnet having a magnetic force of 3000 G and asolenoid having a magnetic force of 1500 G. After drying, the coatedfilm was calendered through 7 stages of metallic rolls at 90° C. and cutto a width of 8 mm to prepare a 8 mm-video tape sample. The resultingsample was designated 2-1.

Samples 2-2 to 2-10 and Comparative Samples 2'-1 to 2'-7 were preparedin the same manner as for Sample 2-1, except for making alterationsshown in Tables 4 and 5 below. Each sample was evaluated according tothe following test methods, and the results obtained are shown in Tables4 and 5.

Volume Ratio (Packing) of Inorganic Powder in Layer (a):

A sample was sliced with a diamond cutter to prepare a specimen having athickness of about 0.1 μm. The specimen was photographed under TEM. Thenumber of inorganic powder particles per μm² was counted, and the numberof the particles contained per unit volume was calculated taking thethickness of the specimen into consideration. Then, the volume perparticle obtained from the same micrograph was multiplied by the numberof the particles per unit volume to obtain a volume ratio of theinorganic powder according to equation:

    Volume Ratio (%)=4π/3(D/2).sup.3 (n/t)×100

wherein D is a particle diameter (μm) obtained from the micrograph; n isa number of particles per unit area (μm²) obtained from the micrograph;and t is a thickness (μm) of the specimen.

Volume Ratio of Powders in Layer (b)

A density of a ferromagnetic powder can be obtained from:

    dM=Bm/4πσ.sub.2

wherein Bm is a residual magnetic flux density (gauss); dM is aferromagnetic powder density (g/cc); and σ₂ is a magnetization of aferromagnetic powder.

A volume ratio of a ferromagnetic powder in layer (b) can be obtained bydividing the above-described powder density by a specific gravity of thepowder.

A volume ratio of other powders in layer (b) can be obtained bycalculating the respective density from the above-described compositionformulation and dividing by the respective specific gravity.

A powder volume ratio in layer (b) can be calculated by adding togetherthese values.

Average Particle Size of Inorganic Powder

An average major axis length was measured with TEM.

Crystallite Size of Ferromagnetic Powder

Obtainable from a half-value width of the X-ray diffraction pattern ofthe (1,1,0) face and (2,2,0) face.

Surface Roughness R_(rms)

Obtainable by scanning over an area of 6 μm×6 μm with a scanning tunnelmicroscope (STM) "Nanoscope II" manufactured by Digital Instrument Co.at a tunnel current of 10 A and a bias voltage of 400 mV. Calculationwas made according to equation: ##EQU3## wherein l is a measured length.Standard Deviation of d (σ)

Measured in the same manner as described in Example 1.

7 MHz Output

Signals of 7 MHz were recorded on a sample tape using a 8 mm video deck"FUJI X8" manufactured by Fuji Photo Film Co., Ltd. The output signalswere determined with an oscilloscope. 8-mm Tape "SAG P6-120" produced byFuji Photo Film Co., Ltd. was used as a reference sample.

Pinholes

A magnetic layer of a sample before formation of a back layer wasobserved with transmitted white light with naked eye to count the numberof pinholes per 100 m². A satisfactory level is 1 per 100 m².

                                      TABLE 4                                     __________________________________________________________________________               Sample No.                                                                    2-1 2-2                                                                              2-3 2-4                                                                              2-5 2'-1                                                                             2'-2                                                                              2'-3                                      __________________________________________________________________________    Layer (a):                                                                    Inorganic Powder:                                                             Kind       TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                 Average particle                                                                         0.04                                                                              0.08                                                                             0.01                                                                              0.04                                                                             0.04                                                                              0.1                                                                              0.006                                                                             0.04                                      size (μm)                                                                  Volume ratio (%)                                                                         45.35                                                                             45.35                                                                            45.35                                                                             45.35                                                                            45.35                                                                             45.35                                                                            45.35                                                                             45.30                                     Amount added (part)                                                                      90  90 90  90 90  90 90  90                                        Carbon Black (part)                                                                      10  10 10  10 10  10 10  10                                        Layer (b):                                                                    Crystallize size of                                                                      0.02                                                                              0.02                                                                             0.02                                                                              0.02                                                                             0.02                                                                              0.02                                                                             0.02                                                                              0.02                                      ferromagnetic powder                                                          (μm)                                                                       Thickness d (μm)                                                                      0.5 0.5                                                                              0.5 0.3                                                                              1   0.5                                                                              0.5 1.2                                       Δd (μm)                                                                         0.11                                                                              0.09                                                                             0.09                                                                              0.12                                                                             0.13                                                                              0.26                                                                             0.27                                                                              0.21                                      Size Ratio*                                                                              2   4  0.5 2  2   5  0.3 2                                         Coating System                                                                           successive wet-on-wet coating system                               Evaluation:                                                                   R.sub.rms (nm)                                                                           6.8 7.1                                                                              7.2 6.8                                                                              6.5 17.3                                                                             18  8.2                                       d/R.sub.rms                                                                              73.5                                                                              70.4                                                                             69.4                                                                              44.1                                                                             153.8                                                                             28.9                                                                             27.8                                                                              146.3                                     7 MHz Output (dB)                                                                        7.5 7.1                                                                              7   7.9                                                                              7   2.2                                                                              0.2 4.5                                       Pinholes (/100 m.sup.2)                                                                  0   0  0   0  0   0  0   0                                         __________________________________________________________________________     Note: *Ratio of inorganic powder average particle size to ferromagnetic       powder crystallite size                                                  

                                      TABLE 5                                     __________________________________________________________________________              Sample                                                                            Sample                                                                            Sample                                                                            Sample                                                                            Sample                                                                            Sample                                                                            Sample                                                                            Sample                                            2'-4                                                                              2-6 2-7 2'-5                                                                              2-8 2'-6                                                                              2'-7                                                                              2-9                                     __________________________________________________________________________    Layer (a):                                                                    Inorganic Powder:                                                             Kind          TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                               Average particle                                                                            0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                                                                              0.04                                    size (μm)                                                                  Volume ratio (%)                                                                            48.87                                                                             39.50                                                                             38.38                                                                             20.97                                                                             19.62                                                                             63.24                                                                             57.76                                   Amount added (part)                                                                         100 60  55  15  12  300 200                                     Carbon Black (part)                                                                         0   40  45  5.5 6   10  10                                      Layer (b):                                                                    Crystallize size of                                                                     0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                                                              0.02                                    ferromagnetic powder                                                          (μm)                                                                       Thickness d (μm)                                                                     0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5                                     Δd (μm)                                                                        --  0.11                                                                              0.18                                                                              0.28                                                                              0.27                                                                              0.31                                                                              --  0.10                                    Size Ratio*                                                                             --  2   2   2   2   2   2   2                                       Coating System                                                                          single                                                                            successive wet-on-wet coating system                                      layer                                                               Evaluation:                                                                   R.sub.rms (nm)                                                                          20.0                                                                              6.2 15  18.2                                                                              7.2 22  failure                                                                           7.1                                     d/R.sub.rms                                                                             25.0                                                                              80.6                                                                              33.3                                                                              27.3                                                                              69.4                                                                              22.7                                                                              of  70.4                                    7 MHz Output (dB)                                                                       -1.5                                                                              7.8 5.2 1.2 7.3 -0.5                                                                              coating                                                                           6.6                                     Pinholes (/100 m.sup.2)                                                                 154 1   0   0   0   0       0                                       __________________________________________________________________________     Note: *The same as the footnote of Table 4.                              

As can be seen from the results in Tables 4 and 5, every sample of thepresent invention, in which the average particle size of the inorganicpowder in layer (a) is from 1/2 to 4 times the crystallite size of theferromagnetic powder in layer (b), has a uniform magnetic layer withsuch excellent surface properties as having a small surface roughnessR_(rms), a d/R_(rms) ratio of 30 or more, and a σ of 0.2 μm or less,achieves a high reproduction output, and is free from pinholes. To thecontrary, Comparative Samples 2'-1 having a size ratio (inorganic powderaverage particle size/ferromagnetic powder crystallite size) greaterthan the prescribed range has a large R_(rms) and shows no improvementin reproduction output. Comparative Example 2'-3 whose magnetic layerhas a large thickness as 1.2 μm is slightly inferior in reproductionoutput. Comparative Example 2'-4 is a single-layered magnetic recordingmedium having no non-magnetic lower layer. Comparative Example 2'-5having a small inorganic powder content has a large R_(rms) and a σ of0.2 or more and shows no improvement in electromagnetic characteristics.Comparative Example 2'-6 having too a small inorganic powder volumeratio has a large R_(rms), failing to obtain improved electromagneticcharacteristics. Comparative Sample 2'-7 encountered a failure ofcoating due to too a high inorganic powder volume ratio.

EXAMPLE 3

Coating Composition for Layer (a)

The same as in Sample 2-1 of Example 2.

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Co-Doped γ-Fe.sub.2 O.sub.3 (Hc: 700 Oe; BET                                                     100    parts                                         specific surface area: 42 m.sup.2 ;                                           crystallite size: 300 Å; σ.sub.s : 75 emu/g)                        Vinyl chloride copolymer (-SO.sub.3 Na                                                                 9      parts                                         content: 1 × 10.sup.-5 eq/g; polymeri-                                  zation degree: 300)                                                           Fine abrasive particles (Cr.sub.2 O;                                                                   7      parts                                         average particle size: 0.3 μm)                                             Toluene                  30     parts                                         Methyl ethyl ketone      30     parts                                         ______________________________________                                    

The above components were kneaded in a kneader for about 1 hour. To themixture were added the following components, followed by furtherdispersing in a kneader for about 2 hours.

    ______________________________________                                        Polyester polyurethane resin (neopentyl-                                                               5 parts                                              glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g; average                       molecular weight: 35000)                                                      Toluene                 200 parts                                             Methyl ethyl ketone     200 parts                                             ______________________________________                                    

To the dispersion were further added the following components, followedby further dispersing in a sand grinder.

    ______________________________________                                        Carbon black (average particle size:                                                                  5 parts                                               20 to 30 mμ; "Ketjenblack EC" produced                                     by Akizo Chemie Nederland B.V.)                                               Coarse abrasive grains (α-alumina;                                                              2 parts                                               "AKP-12" produced by Sumitomo Chemical                                        Co., Ltd.; average particle size: 0.5 μm)                                  ______________________________________                                    

Finally, the following composition was added to the dispersion, followedby further dispersing in a sand grinder to prepare a coating compositionfor layer (b).

    ______________________________________                                        Polyisocyanate (Coronate L)                                                                       6 parts                                                   Tridecyl stearate   6 parts                                                   ______________________________________                                    

The coating composition for layer (a) was coated on a 75 μm thickpolyethylene terephthalate film support to a dry thickness of 1.5 μm,and the coating composition for layer (b) was then coated thereon to adry thickness of 0.5 μm while layer (a) was wet. The back side of thesupport was coated in the same manner. The coated film was subjected tocalendering to obtain a magnetic recording medium.

The resulting magnetic recording medium was punched out to obtain a discof 3.5 in. in diameter. The disc was put in a 3.5 in. cartridgepreviously having a liner therein, and the cartridge was fitted withprescribed members to obtain a 3.5 in. floppy disc. The resulting samplewas designated 3-1.

Samples 3-2 to 3-5 and Comparative Samples 3'-1 to 3'-3 were prepared inthe same manner as for Sample 3-1, except for making the alterationsshown in Table 6 below. Each sample was evaluated as follows, and theresults obtained are shown in Table 6.

Volume Ratio (Packing) of Inorganic Powder in Layer (a)

The same as in Example 2.

Average Particle Size of Inorganic Powder

The same as in Example 2.

Crystallite Size of Ferromagnetic Powder

Obtainable from the X-ray diffraction pattern of the (4.4.0) face and(2.2.0) face.

Surface Roughness R_(rms)

The same as in Example 2.

Standard Deviation of d (σ)

The same as in Example 1.

Innermost Periphery 2F Output

Relatively expressed by taking the initial 2F output of Sample 3-1 was100%. A drive "PD 211" manufactured by Toshiba was used.

                                      TABLE 6                                     __________________________________________________________________________               Sample No.                                                                    3-1 3-2                                                                              3-3 3-4                                                                              3-5 3'-1                                                                             3'-2                                                                              3'-3                                      __________________________________________________________________________    Layer (a):                                                                    Inorganic Powder:                                                             Kind       TiO.sub.2                                                                         Tid2                                                                             TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                 Average particle                                                                         0.04                                                                              0.09                                                                             0.02                                                                              0.04                                                                             0.04                                                                              0.13                                                                             0.01                                                                              0.04                                      size (μm)                                                                  Volume ratio (%)                                                                         45.35                                                                             45.35                                                                            45.35                                                                             45.35                                                                            45.35                                                                             45.35                                                                            45.35                                                                             45.35                                     Amount added (part)                                                                      90  90 90  90 90  90 90  90                                        Carbon Black (part)                                                                      10  10 10  10 10  10 10  10                                        Layer (b):                                                                    Crystallize size of                                                                      0.03                                                                              0.03                                                                             0.03                                                                              0.03                                                                             0.03                                                                              0.03                                                                             0.03                                                                              0.03                                      ferromagnetic powder                                                          (μm)                                                                       Thickness d (μm)                                                                      0.5 0.5                                                                              0.5 0.3                                                                              1   0.5                                                                              0.5 1.2                                       Δd (μm)                                                                         0.09                                                                              0.08                                                                             0.11                                                                              0.061                                                                            0.21                                                                              0.31                                                                             0.28                                                                              0.12                                      Size Ratio*                                                                              1.33                                                                              3.00                                                                             0.67                                                                              1.33                                                                             1.33                                                                              4.33                                                                             0.33                                                                              1.33                                      Coating System                                                                           successive wet-on-wet coating system                               Evaluation:                                                                   R.sub.rms (nm)                                                                           7.5 8.1                                                                              7.6 8.5                                                                              7.3 17.2                                                                             18.9                                                                              7.6                                       d/R.sub.rms                                                                              66.7                                                                              61.7                                                                             65.8                                                                              35.3                                                                             137.0                                                                             29.1                                                                             26.5                                                                              157.9                                     Relative 2F Output                                                                       100 95 101 94 92  75 68  72                                        (%)                                                                           __________________________________________________________________________     Note: *The same as the footnote of Table 4.                              

As is apparent from Table 6, every sample of the present invention, inwhich the average particle size of the inorganic powder in layer (a) isfrom 1/2 to 4 times the crystallite size of the ferromagnetic powder inlayer (b), has a uniform magnetic layer with such excellent surfaceproperties as having a small surface roughness R_(rms), a d/R_(rms)ratio of 30 or more, and a σ of 0.2 μm or less, achieves a highreproduction output, and is free from pinholes. To the contrary,Comparative Sample 3'-1 having a size ratio (inorganic powder averageparticle size/ferromagnetic powder crystallite size) greater than theprescribed range has a large R_(rms) and shows no improvement inreproduction output. Comparative Sample 3'-2 having the size ratiosmaller than the presecribed range also has a large R_(rms) and shows noimprovement in reproduction output. Comparative Sample 3'-3 whosemagnetic layer has a large thickness as 1.2 μm is slightly inferior inreproduction output.

EXAMPLE 4

Magnetic recording medium samples (Samples 4-1 to 4-9, ComparativeSamples 4'-1 to 4'-6, and Reference Example 4"-1) were prepared in thesame manner as for Sample 2-1 of Example 2, except for makingalterations particularly to the ratio of inorganic powder averageparticle size to ferromagnetic powder major axis length as shown inTables 7 and 8 below. Each sample was evaluated in the same manner as inExample 2. In addition, a squareness ratio was determined as follows.The results obtained are shown in Tables 7 and 8.

Squareness Ratio (Rs)

A Br/Bm ratio in the coating direction was measured in a magnetic fieldof 5 kOe by means of a vibrating sample magnetometer ("VNM" manufacturedby Toei Kogyo K. K.).

                                      TABLE 7                                     __________________________________________________________________________                Sample No.                                                                    4-1 4-2                                                                              4-3 4-4                                                                              4-5 4'-1                                                                             4'-2                                                                             4'-3                                      __________________________________________________________________________    Layer (a):                                                                    Inorganic Powder:                                                             Kind        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                    Average particle size (μm)                                                             0.035                                                                             0.065                                                                            0.035                                                                             0.035                                                                            0.035                                                                             0.008                                                                            0.035                                        Volume ratio (%)                                                                          45.35                                                                             45.35                                                                            45.35                                                                             39.50                                                                            48.87                                                                             45.35                                                                            45.35                                        Amount added (part)                                                                       90  90 90  60 100 90 90                                           Carbon Black (part)                                                                       10  10 10  40 0   10 10                                           Layer (b):                                                                    Major axis length of                                                                      0.2 0.2                                                                              0.2 0.2                                                                              0.2 0.2                                                                              0.2                                                                              0.2                                       ferromagnetic powder (μm)                                                  Thickness d (μm)                                                                       0.5 0.5                                                                              1   0.5                                                                              0.5 0.5                                                                              1.3                                                                              0.5                                       Δd (μm)                                                                          0.077                                                                             0.075                                                                            0.074                                                                             0.077                                                                            0.085                                                                             0.31                                                                             0.09                                                                             --                                        Size Ratio* 0.175                                                                             0.325                                                                            0.175                                                                             0.175                                                                            0.175                                                                             0.4                                                                              0.175                                                                            0                                         Coating System                                                                            successive wet-on-wet coating system                                                                  single                                                                        layer                                     Evaluation:                                                                   R.sub.rms (nm)                                                                            6.6 6.8                                                                              5.4 15 7.2 25 8.8                                                                              33                                        d/R.sub.rms 75.8                                                                              73.5                                                                             185 33.3                                                                             69  20 147                                                                              15.2                                      Rs          0.84                                                                              0.81                                                                             0.86                                                                              0.79                                                                             0.76                                                                              0.66                                                                             0.81                                                                             0.81                                      7 MHz Output (dB)                                                                         6.8 6.5                                                                              6.5 5.9                                                                              6.8 4.4                                                                              3.5                                                                              -2.2                                      Pinholes (/100 m.sup.2)                                                                   0   0  0   0  0   0  0  153                                       __________________________________________________________________________     Note: *Ratio of inorganic powder average particle size to ferromagnetic       powder major axis length.                                                

                                      TABLE 8                                     __________________________________________________________________________                Sample No.                                                                    4'-4  4-6   4"-1                                                                             4-7   4-8   4-9   4'-5  4'-6                       __________________________________________________________________________    Layer (a):                                                                    Inorganic Powder:                                                             Kind        TiO.sub.2                                                                           α-Fe.sub.2 O.sub.3                                                               α-Al.sub.2 O.sub.3                                                            BaSO.sub.4                                                                          SiO.sub.2   TiO.sub.2                  Average particle size (μm)                                                             0.035 0.04     0.06  0.033 0.02        0.035                      Volume ratio (%)                                                                          45.35 42.37    46.50 44.98 53.33 34.84 38.88                      Amount added (part)                                                                       90    90       90    90    90    0     55                         Carbon Black (part)                                                                       10    10       10    10    10    100   45                         Layer (b):                                                                    Major axis length of                                                                      0.2   0.2   0.2                                                                              0.2   0.2   0.2   0.2   0.2                        ferromagnetic powder (μm)                                                  Thickness d (μm)                                                                       0.5   0.5   2.5                                                                              0.5   0.5   0.5   0.5   0.5                        Δd (μm)                                                                          0.01  0.055 -- 0.09  0.18  0.05  0.29  0.33                       Size Ratio* 0.175 0.2   -- 0.3   0.165 0.1   0.175 0.175                      Coating System                                                                            successive                                                                          successive                                                                          single                                                                           successive                                                                          successive                                                                          successive                                                                          successive                                                                          successive                             wet-on-dry                                                                          wet-on-wet                                                                          layer                                                                            wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                 Evaluation:                                                                   R.sub.rms (nm)                                                                            15.5  14.3  9.5                                                                              8.8   11.5  9.6   55.3  48.2                       d/R.sub.rms 32.3  35    2600                                                                             57    43    52    9     10.3                       Rs          0.88  0.82  0.77                                                                             0.77  0.75  0.71  0.68  0.69                       7 MHz Output (dB)                                                                         6.8   6.5   6.5                                                                              5.9   6.8   4.4   3.5   -2.2                       Pinholes (/100 m.sup.2)                                                                   0     0     0  0     0     0     0     153                        __________________________________________________________________________     Note: *The same as the footnote of Table 7.                              

It is apparent from Tables 7 and 8 that every sample of the presentinvention, in which the average particle size of the inorganic powder inlayer (a) is not more than 1/3 the major axis length of theferromagnetic powder in layer (b), has a uniform magnetic layer withsuch excellent surface properties as having a d/R_(rms) ratio of 30 ormore and a .sup.Δ d of not more than d/2, achieves a high reproductionoutput, and is free from pinholes. To the contrary, Comparative Samples4'-1 having a size ratio (inorganic powder average particlesize/ferromagnetic powder major axis length) greater than the prescribedrange has a large Ad and shows no improvement in reproduction output.Comparative Sample 4'-3 having a single layer structure lacks filmthickness and is seriously inferior in coating properties andelectromagnetic characteristics. Comparative Sample 4'-4 which wasprepared by coating layer (b) after drying of layer (a) fails to obtainimprovements in coating properties. Reference Sample 4"-1, an examplehaving a single magnetic layer structure having a large thickness d,exhibits satisfactory coating properties but shows no improvement inelectromagnetic characteristics. Comparative Sample 4'-5 using noinorganic powder in layer (a) suffers from a considerable interfacialvariation between layers (a) and (b) (i.e., .sup.Δ d is high) andtherefore has poor electromagnetic characteristics. Comparative Sample4'-6 has a low volume ratio of the inorganic powder in layer (a) and aslightly increased Ad and therefore poor electromagneticcharacteristics.

EXAMPLE 5

A magnetic recording medium was prepared in the same manner as forSample 3-1, except for replacing the Co-doped γ-Fe₂ O₃ of theformulation for layer (b) with the following ferromagnetic powder. Theresulting sample was designated 5-1.

Ferromagnetic Powder

Hexagonal barium ferrite

Average plate diameter: 0.05 μm

Average aspect ratio: 4

BET specific surface area: 39 m² /g

Hc: 1100 Oe

Samples 5-2 to 5-3 and Comparative Samples 5'-1 to 5'-2 were prepared inthe same manner as for Sample 5-1, except for making alterationsparticularly to the size ratio (inorganic powder average particlesize/ferromagnetic powder average plate diameter) as shown in Table 9below.

Each of these samples was evaluated in the same manner as in Example 3.In addition, the following properties were also determined. The resultsobtained are shown in Table 9.

Squareness Ratio (Rs) in Vertical Direction

A Br/Bm ratio in the direction perpendicular to the coated surface wasobtained with a vibrating sample magnetometer.

D₅₀

A recording density (D₅₀) (kfci) which makes the output 50% of that oflong wavelength recording reproduction was obtained. D₅₀ affords anindication of the highest possible recording density which may bereached with a particular recording device used.

                                      TABLE 9                                     __________________________________________________________________________                Sample No.                                                                    5-1   5'-1  5'-2                                                                             5-2   5-3                                          __________________________________________________________________________    Layer (a):                                                                    Inorganic Powder:                                                             Kind        TiO.sub.2                                                                           TiO.sub.2                                                                              TiO.sub.2                                                                           TiO.sub.2                                    Average particle size (μm)                                                             0.035 0.06     0.035 0.06                                         True specific gravity                                                                     4.3   4.3      4.3   4.3                                          Volume ratio (%)                                                                          45.80 45.80    41.89 45.80                                        Amount added (part)                                                                       90    90       70    90                                           Carbon Black (part)                                                                       10    10       30    10                                           Layer (b):                                                                    Plate diameter (μm)*                                                                   0.05  0.05  0.05                                                                             0.05  0.09                                         d (μm)   0.5   0.5   1.2                                                                              0.5   0.5                                          Δd (μm)                                                                          0.12  0.29     0.10  0.15                                         Size Ratio**                                                                              0.7   1.2   0  0.7   0.66666                                      Coating System                                                                            successive                                                                          successive                                                                          single                                                                           successive                                                                          successive                                               wet-on-wet                                                                          wet-on-wet                                                                          layer                                                                            wet-on-wet                                                                          wet-on-wet                                   Evaluation:                                                                   R.sub.rms (nm)                                                                            8.7   18.8  12.3                                                                             11.6  12.9                                         d/R.sub.rms 57    26.6  975                                                                              43.1  38.8                                         Rs          0.72  0.55  0.75                                                                             0.72  0.71                                         Rs in vertical direction                                                                  0.75  0.72  0.65                                                                             0.73  0.71                                         D.sub.50 (kfci)                                                                           40    25    15 38    42                                           Pinholes (/100 m.sup.2)                                                                   0     0     0  0     0                                            __________________________________________________________________________     Note:                                                                         *Ferromagnetic powder                                                         **Ratio of inorganic powder average particle size to ferromagnetic powder     plate diameter                                                           

As can be seen from Table 9, since the inorganic powder used in layer(a) of the samples of the present invention has an average particlediameter smaller than the average plate diameter of the tabularferromagnetic powder in layer (b), a uniform magnetic layer having a Adof not more than d/2 and having a high squareness ratio in the verticaldirection can be formed. Therefore, these samples have a high D₅₀ and isfree from pinholes. To the contrary, Comparative Sample 5'-1, in whichthe inorganic powder has an average particle size greater than theaverage plate diameter of the ferromagnetic powder, has a large .sup.Δd, showing no improvement on D₅₀. Comparative Sample 5'-2, which is anexample of a single layer structure having no lower non-magnetic layer,has a poor D₅₀.

EXAMPLE 6

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Rutile TiO.sub.2 (average particle size:                                                               80     parts                                         0.035 μm; TiO.sub.2 content: 90%; surface                                  treating agent: Al.sub.2 O.sub.3 (10%); BET                                   specific surface area: 40 m.sup.2 /g; DBP                                     absorption: 27 to 38 g/100 g; pH: 7)                                          Carbon black (average particle size:                                                                   20     parts                                         16 mμ; DBP absorption: 80 ml/100 g;                                        pH: 8.0; BET specific surface area:                                           250 m.sup.2 /g; volatile content: 1.5%)                                       Vinyl chloride-vinyl acetate-vinyl                                                                     12     parts                                         alcohol copolymer (86:13:1; -N(CH.sub.3).sub.3.sup.+ Cl.sup.-                 content: 5 × 10.sup.-6 eq/g; polymerization                             degree: 400)                                                                  Polyester polyurethane resin (neopentyl-                                                               5      parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               Butyl stearate           1      part                                          Stearic acid             1      part                                          Methyl ethyl ketone      100    parts                                         Cyclohexanone            50     parts                                         Toluene                  50     parts                                         ______________________________________                                    

The above components were kneaded in a continuous kneader and dispersedin a sand mill. To the dispersion were added 1 part of polyisocyanateand 40 parts of butyl acetate, followed by filtration through a filterhaving an average pore size of 1 μm to prepare a coating composition forlayer (a).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: Fe/Zn/Ni alloy                                                                   100    parts                                         (92/4/4; Hc: 1600 Oe; BET specific                                            surface area: 60 m.sup.2 /g; crystallite                                      size: 195 Å; average major axis                                           length: 0.20 μm; acicular ratio: 10;                                       σ.sub.s: 130 emu/g; surface treating agent:                             Al.sub.2 O.sub.3, SiO.sub.2)                                                  Vinyl chloride copolymer (-SO.sub.3 Na content:                                                        12     parts                                         1 × 10.sup.-4 eq/g; polymerization degree: 300)                         Polyester polyurethane resin (neopentyl-                                                               3      parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               α-Alumina (average particle size: 0.3 μm)                                                     2      parts                                         Carbon black (average particle size:                                                                   0.5    pats                                          0.10 mμ)                                                                   Butyl stearate           1      part                                          Stearic acid             2      parts                                         Methyl ethyl ketone      90     parts                                         Cyclohexanone            50     parts                                         Toluene                  60     parts                                         ______________________________________                                    

The above components were kneaded in a continuous kneader and dispersedin a sand mill. To the dispersion were added 3 parts of polyisocyanateand 40 parts of butyl acetate, followed by filtration through a filterhaving an average pore size of 1 μm to prepare a coating composition forlayer (b).

The coating composition for layer (a) was coated on a 7 μm thickpolyethylene terephthalate film support having a centerline averagesurface roughness of 0.01 μm to a dry thickness of 2 μm. Immediatelythereafter while layer (a) was wet, the coating composition for layer(b) was coated thereon to a dry thickness of 0.5 μm (successivewet-on-wet coating). While layers (a) and (b) were wet, theferromagnetic powder in layer (b) was orientated by applying a magneticfield using a cobalt magnet having a magnetic force of 3000 G and asolenoid having a magnetic force of 1500 G. After drying, the coatedfilm was calendered through 7 stages of metallic rolls at 90° C. and cutto a width of 8 mm to prepare a 8 mm-video tape sample. The resultingsample was designated 6-1.

Samples 6-2 to 6-12 and Comparative Samples 6'-1 to 6'-3 were preparedin the same manner as for Sample 6-1, except for making alterationsshown in Tables 10 and 11 below. Electromagnetic characteristics of theresulting samples were evaluated in the same manner as in the foregoingexamples. In addition, centerline surface roughness (Ra) of layer (b)was measured over an area of about 250×250 nm according to a MIRAUmethod by means of "TOYO 3D" manufactured by WYKO K. K. Sphericalcorrections and cylindrical corrections were made at about 650 nm. Theresults obtained are shown in Tables 10 and 11. The relative runningspeed of the 8 mm VTR used for testing was 38 m/sec, and the 7 MHzrecording wavelength was 0.54 μm. Accordingly, λ/50 was 10.8 nm.

                  TABLE 10                                                        ______________________________________                                                  Sample No.                                                                    6-1  6-2    6-3    6-4  6-5  6-6  6-7                               ______________________________________                                        Layer (a):                                                                    Inorganic powder                                                                          TiO.sub.2                                                                            TiO.sub.2                                                                            TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                   rutile rutile rutile                                                                             rutile                                                                             rutile                                                                             rutile                                                                             rutile                          Surface coat:                                                                 Al.sub.2 O.sub.3 (%)                                                                      10     0      0    5    5    0    5                               SiO.sub.2 (%)                                                                             0      5      0    3    0    1    3                               ZrO.sub.2 (%)                                                                             0      0      5    0    1    1    0.5                             Main component (%)                                                                        90     90     91   90   88   91   85                              Al.sub.2 O.sub.3 /surface coat                                                            1.000  0.000  0.000                                                                              0.625                                                                              0.833                                                                              0.000                                                                              0.588                           Thickness (μm)                                                                         2.5    2.5    2.5  2.5  2.5  2.5  2.5                             Layer (b):                                                                    d (μm)   0.5    0.5    0.5  0.5  0.5  0.5  0.5                             σ (μm)                                                                           0.09   0.10   0.08 0.06 0.05 0.11 0.15                            Δd (μm)                                                                          0.16   0.11   0.18 0.08 0.21 0.21 0.12                            Coating System                                                                            successive wet-on-wet coating                                     Ra (nm)     3.5    7.6    4.1  3.5  3.2  8.8  2.7                             7 MHz Output (dB)                                                                         6.5    6.6    5.5  6.6  6.7  6.3  6.4                             C/N (dB)    6.1    6      5.9  6    5.9  6.1  6.1                             ______________________________________                                    

                                      TABLE 11                                    __________________________________________________________________________              Sample No.                                                                    6'-1  6-8   6'-2  6'-3  6-9   6-10  6-11  6-12                      __________________________________________________________________________    Layer (a):                                                                    Inorganic powder                                                                        TiO.sub.2                                                                           TiO.sub.2                                                                           TiO.sub.2                                                                           TiO.sub.2                                                                           TiO.sub.2                                                                           α-hematite                                                                    BaSO.sub.4                                                                          ZnO                                 rutile                                                                              rutile                                                                              rutile                                                                              rutile                                                                              anatase                                     Surface coat:                                                                 Al.sub.2 O.sub.3 (%)                                                                    0     10    10    10    10    8     5     5                         SiO.sub.2 (%)                                                                           0     0     0     0     0     2     5     3                         ZrO.sub.2 (%)                                                                           0     0     0     0     0     0     0     0                         Main component (%)                                                                      98    90    90    90    90    90    90    90                        Al.sub.2 O.sub.3 /surface coat                                                          0.000 1.000 1.000 1.000 1.000 0.800 0.500 0.625                     Thickness (μm)                                                                       2.5   2.5   2.5   2.5   2.5   2.5   2.5   2.5                       Layer (b):                                                                    d (μm) 0.5   1     1.2   0.5   1     0.5   0.5   0.5                       σ (μm)                                                                         0.22  0.18  0.41        0.11  0.06  0.06  0.05                      Δd (μm)                                                                        0.28  0.22  0.46  --    0.21  0.15  0.08  0.16                      Coating System                                                                          successive                                                                          successive                                                                          successive                                                                          successive                                                                          successive                                                                          successive                                                                          successive                                                                          successive                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-dry                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                Ra (nm)   12.1  5.5   3.6   failure                                                                             8.9   4.3   6.3   9.1                                                   of sample                                                                     prepa-                                                                        ration                                            7 MHz Output (dB)                                                                       2.5   5.7   4.5   --    6     5.9   5.8   6                         C/N (dB)  2.1   5.9   3.8   --    5.8   5.5   5.5   6                         __________________________________________________________________________

As is apparent from Tables 10 and 11, the inorganic particles used inSamples 6-1 to 6-12 have improved dispersibility owing to the surfacecoat comprising Al₂ O₃, SiO₂, ZrO₂, etc. to thereby achieve a low Ra,not more than λ/50 (10.8 nm), and exhibit satisfactory electromagneticcharacteristics. Having no surface coat on the inorganic powder,Comparative Sample 6'-1 has poor dispersibility and, as a result, hashigh Ra, σ, and .sup.Δ d and deteriorated electromagneticcharacteristics. Comparative Sample 6'-2 has poor electromagneticcharacteristics due to the large thickness of the magnetic layer.Comparative Sample 6'-3 could not be prepared because of a failure ofsuccessive wet-on-dry coating.

EXAMPLE 7

Coating Composition for Layer (a)

The same as for Sample 6-1 of Example 6.

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Co-Substituted barium ferrite                                                                         100    parts                                          (BET specific surface area: 35 m.sup.2 /g;                                    average particle size: 0.06 μm;                                            aspect ratio: 5)                                                              Vinyl chloride copolymer (-SO.sub.3 Na                                                                9      parts                                          content: 1 × 10.sup.-5 eq/g; polymeri-                                  zation degree: 300)                                                           Fine abrasive grins (Cr.sub.2 O; average                                                              7      parts                                          particle size: 0.3 μm)                                                     Toluene                 30     parts                                          Methyl ethyl ketone     30     parts                                          ______________________________________                                    

The above components were kneaded in a kneader for about 1 hour. To theresulting mixture was added the following composition, followed byfurther dispersing in a kneader for about 2 hours.

    ______________________________________                                        Polyester polyurethane resin                                                                          5      parts                                          (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g; average                                    molecular weight: 35000)                                                      Methyl ethyl ketone     200    parts                                          Cyclohexanone           100    parts                                          Toluene                 80     parts                                          ______________________________________                                    

Then, the following components were further added thereto, followed bydispersing in a sand grinder at 2000 rpm for about 2 hours.

    ______________________________________                                        Carbon black (average particle                                                                        5 parts                                               size: 20 to 30 mμ; "Ketjenblack EC"                                        produced by Akizo Chemie Nederland B.V.)                                      Coarse abrasive grains (α-alumina;                                                              2 parts                                               "AKP-12" produced by Sumitomo Chemical                                        Co., Ltd.; average particle size: 0.5 μm)                                  ______________________________________                                    

Finally, the following composition was added to the dispersion, followedby further dispersing in a sand grinder to prepare a coating compositionfor layer (b).

    ______________________________________                                        Polyisocyanate (Coronate L)                                                                       6 parts                                                   Tridecyl stearate   6 parts                                                   ______________________________________                                    

The coating composition for layer (a) was coated on a 75 μm thickpolyethylene terephthalate film support to a dry thickness of 1.5 μm,and the coating composition for layer (b) was then coated thereon to adry thickness of 0.5 μm while layer (a) was wet. The back side of thesupport was coated in the same manner. The coated film was subjected tocalendering to obtain a magnetic recording medium.

The resulting magnetic recording medium was punched out to obtain a discof 3.5 in. in diameter. The disc was put in a 3.5 in. cartridgepreviously having a liner therein, and the cartridge was fitted withprescribed members to obtain a 3.5 in. floppy disc. The resulting samplewas designated 7-1.

Samples 7-2 to 7-7 and Comparative Sample 7'-1 were prepared in the samemanner as for Sample 7-1, except for making the alterations shown inTable 12 below.

Each sample was evaluated in terms of surface roughness Ra in the samemanner as in Example 6 and in terms of initial innermost periphery 2Foutput. The initial 2F output was relatively expressed by taking that ofSample 7-1 was 100%. A drive "PD 211" manufactured by Toshiba was used.The recording wavelength was 1.428 μm, leading to λ/50=28.5 nm. Theresults obtained are shown in Table 12.

                                      TABLE 12                                    __________________________________________________________________________               Sample No.                                                                    7-1 7-2                                                                              7-3 7-4                                                                7-5               7-6                                                                              7-7 7'-1                                      __________________________________________________________________________    Layer (a):                                                                    Inorganic powder                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                                                        TiO.sub.2                                                                         TiO.sub.2                                            rutile                                                                            rutile                                                                           rutile                                                                            rutile                                                                           rutile                                                                            rutile                                                                           rutile                                                                            rutile                                    Surface coat:                                                                 Al.sub.2 O.sub.3 (%)                                                                     10  0  0   5  5   0  5   0                                         SiO.sub.2 (%)                                                                            0   5  0   3  0   1  3   0                                         ZrO.sub.2 (%)                                                                            0   0  5   0  1   1  0.5 0                                         Main component (%)                                                                       90  90 91  90 88  91 85  98                                        Al.sub.2 O.sub.3 /surface coat                                                           1.000                                                                             0.000                                                                            0.000                                                                             0.625                                                                            0.833                                                                             0.000                                                                            0.588                                                                             0.000                                     Thickness (μm)                                                                        2.5 2.5                                                                              2.5 2.5                                                                              2.5 2.5                                                                              2.5 2.5                                       Layer (b):                                                                    d (μm)  0.5 0.5                                                                              0.5 0.5                                                                              0.5 0.5                                                                              0.5 0.5                                       σ (μm)                                                                          0.09                                                                              0.06                                                                             0.07                                                                              0.05                                                                             0.08                                                                              0.07                                                                             0.06                                                                              0.25                                      Δd (μm)                                                                         0.11                                                                              0.16                                                                             0.16                                                                              0.13                                                                             0.09                                                                              0.21                                                                             0.18                                                                              0.28                                      Coating System                                                                           successive wet-on-wet coating                                      Ra (nm)    16  18 22  8.3                                                                              9.0 12 25  31                                        Initial 2F Output (%)                                                                    100 98 103 95 96  101                                                                              102 86                                        __________________________________________________________________________

As is seen from Table 12, similar to Example 6, the inorganic particlesused in the samples according to the present invention have improveddispersibility owing to the surface coat comprising Al₂ O₃, SiO₂, ZrO₂,etc. to thereby achieve a low Ra and exhibit satisfactoryelectromagnetic characteristics. Having no surface coat on the inorganicpowder, Comparative Sample 7'-1 has poor dispersibility and, as aresult, has high Ra, σ, and Ad and deteriorated electromagneticcharacteristics.

EXAMPLE 8

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Rutile TiO.sub.2 (average particle size:                                                               80     parts                                         0.035 μm; TiO.sub.2 content: ≧90%; BET                              specific surface area: 40 m.sup.2 /g;                                         DBP absorption: 27 to 38 g/100 g;                                             pH: 7)                                                                        Carbon black (average particle size:                                                                   20     parts                                         16 mμ; DBP absorption: 80 ml/100 g;                                        pH: 8.0; BET specific surface area:                                           250 m.sup.2 /g; volatile content: 1.5%)                                       Vinyl chloride-vinyl acetate-vinyl                                                                     12     parts                                         alcohol copolymer (86:13:1; -N(CH.sub.3).sub.3.sup.+ Cl.sup.-                 content: 5 × 10.sup.-6 eq/g; polymerization                             degree: 400)                                                                  Polyester polyurethane resin (neopentyl-                                                               5      parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eg/g)                               Butyl stearate           1      part                                          Stearic acid             1      part                                          Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were kneaded in an open kneader and dispersed in asand mill. To the dispersion were added 1 part of polyisocyanate and 40parts of butyl acetate, followed by filtration through a filter havingan average pore size of 1 μm to prepare a coating composition for layer(a).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: Fe/Zn/Ni alloy                                                                   100    parts                                         (92/4/4; Hc: 1600 Oe; BET specific                                            surface area: 60 m.sup.2 /g; crystallite                                      size: 195 Å; average major axis                                           length: 0.20 μm; acicular ratio:                                           7; σ.sub.s : 128 emu/g)                                                 Vinyl chloride copolymer (-SO.sub.3 Na content:                                                        12     parts                                         1 × 10.sup.-4 eq/g; polymerization degree: 300)                         Polyester polyurethane resin (neopentyl-                                                               3      parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               α-Alumina (average particle size: 0.2 μm)                                                     2      parts                                         Carbon black (average particle size:                                                                   0.5    pats                                          0.10 mμ)                                                                   Butyl stearate           1      part                                          Stearic acid             2      parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were kneaded in an open kneader and dispersed in asand mill. To the dispersion were added 3 parts of polyisocyanate and 40parts of butyl acetate, followed by filtration through a filter havingan average pore size of 1 μm to prepare a coating composition for layer(b).

The coating composition for layer (a) was coated on a 7 μm thickpolyethylene terephthalate film support having a centerline averagesurface roughness of 0.01 μm to a dry thickness of 3 μm. Immediatelythereafter while layer (a) was wet, the coating composition for layer(b) was coated thereon to a dry thickness of 0.5 μm (successivewet-on-wet coating). While layers (a) and (b) were wet, theferromagnetic powder in layer (b) was orientated by applying a magneticfield using a cobalt magnet having a magnetic force of 3000 G and asolenoid having a magnetic force of 1500 G. After drying, the coatedfilm was calendered through 7 stages of metallic rolls at 90° C. and cutto a width of 8 mm to prepare a 8 mm-video tape sample. The resultingsample was designated 8-1.

Samples 8-2 to 8-6 and Comparative Samples 8'-1 to 8'-4 were prepared inthe same manner as for Sample 8-1, except for making alterations to theformulation of layer (b) as shown in Table 13 and to the formulation oflayer (a) as shown in Table 14 below. In the preparation of Sample 8-4and Comparative Sample 8'-1, the coating composition for layer (b) wasprepared by using a continuous kneader in order to increase the packingof the ferromagnetic powder.

Each of the resulting samples was evaluated as follows. The resultsobtained are shown in Table 15 below.

7 MHz Output

Signals of 7 MHz were recorded on a sample tape using a 8 mm video deck"FUJI X8" manufactured by Fuji Photo Film Co., Ltd. The output signalswere determined with an oscilloscope. A 8 mm tape "SAG P6-120" producedby Fuji Photo Film Co., Ltd. was used as a reference sample.

C/N

7 MHz signals were recorded on a sample tape using a 8 mm video deck"FUJI X8". The noise generated at 6 MHz on reproduction of the recordedsignals was measured with a spectrum analyzer, and a ratio of thereproduction output to the noise was obtained.

Percent Thermal Shrinkage

An about 100 mm long sample tape was preserved in a thermostat at 70° C.for 48 minutes without any tension, and the percent change in lengthbefore and after the preservation was measured with a comparator.

Skewness

Color bar signals were recorded on a sample tape, and the recorded tapewas preserved in a thermostat at 70° C. for 48 hours. After taking outthe sample therefrom and confirming that the sample temperature returnedto room temperature, the recorded signals were reproduced. Skewness wasdetermined from the change of color bar signals on the reproduced image.One color bar corresponded to 7.48 μsec.

Running Durability

A sample tape in a cassette P6-120 was played 100 passes on ten 8mm-video decks "FUJI X8" in an atmosphere of 23° C. and 70% RH. Areduction in output during 100 passes was measured, and the degree ofcontamination in the inside of the deck was observed after running.Running durability was evaluated according to the following ratingsystem:

Good . . . Five or less contaminated parts are observed.

Medium . . . More than 5 contaminated parts are observed, but causing nojamming.

Bad . . . More than 5 contaminated parts are observed, causing jamming.

"Good to medium" means performance intermediate between "good" and"medium".

                                      TABLE 13                                    __________________________________________________________________________    Formulation of Layer (b)                                                                 Sample No.                                                                    8-1                                                                              8-2                                                                              8-3                                                                              8-4                                                                              8'-1                                                                             8'-2                                                                             8-5                                                                              8'-3                                                                              8-6                                       __________________________________________________________________________    d (μm)  0.5                                                                              0.5                                                                              0.5                                                                              0.5                                                                              0.5                                                                              0.5                                                                              1  1.2 0.1                                       σ (μm)                                                                          0.11                                                                             0.12                                                                             0.11                                                                             0.08                                                                             0.26                                                                             0.28                                                                             0.09                                                                             0.21                                                                              0.15                                      Composition (part):                                                           Ferromagnetic powder                                                                     100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100 100                                       Vinyl chloride                                                                           12 12 12 12 12 12 12 12  12                                        Copolymer                                                                     Urethane   3  3  3  5  5  3  3  3   3                                         Isocyanate 3  3  3  5  5  3  3  3   3                                         α-Alumina                                                                          2  2  8  10 10 2  2  2   2                                         Ferromagnetic Powder:                                                         Bm (gauss) 3200                                                                             3000                                                                             3200                                                                             3200                                                                             3200                                                                             3200                                                                             3200                                                                             3200                                                                              3200                                      σ.sub.s (emu/g)                                                                    130                                                                              130                                                                              129                                                                              121                                                                              121                                                                              130                                                                              130                                                                              130 130                                       dM (g/cc)  1.96                                                                             1.96                                                                             1.85                                                                             2.10                                                                             2.10                                                                             1.96                                                                             1.96                                                                             1.96                                                                              1.96                                      Ferromagnetic Powder                                                                     33.77                                                                            33.77                                                                            31.91                                                                            36.28                                                                            36.28                                                                            33.77                                                                            33.77                                                                            33.77                                                                             33.77                                     Volume Ratio (%)                                                              α-Alumina Volume                                                                   0.98                                                                             0.98                                                                             3.70                                                                             5.26                                                                             5.26                                                                             0.98                                                                             0.98                                                                             0.98                                                                              0.98                                      Ratio (%)                                                                     Total Powder Volume                                                                      34.75                                                                            34.75                                                                            35.61                                                                            41.55                                                                            41.55                                                                            34.75                                                                            34.75                                                                            34.5                                                                              34.75                                     Ratio (%)                                                                     __________________________________________________________________________

                                      TABLE 14                                    __________________________________________________________________________    Formulation of Layer (a)                                                                  Sample No.                                                                    8-1                                                                              8-2                                                                              8-3                                                                              8-4                                                                              8'-1                                                                             8'-2                                                                             8-5                                                                              8'-3                                                                             8-6                                       __________________________________________________________________________    Thickness (μm)                                                                         2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2.5                                                                              2  1.8                                                                              2.5                                       TiO.sub.2 Average                                                                         0.035                                                                            0.035                                                                            0.035                                                                            0.035                                                                            0.035                                                                            0.035                                                                            0.035                                                                            0.035                                                                            0.035                                     Particle Size (μm)                                                         Composition (part):                                                           TiO.sub.2   80 100                                                                              100                                                                              90 90 75 100                                                                              100                                                                              100                                       Carbon black                                                                              20 0  0  10 10 25 0  0  0                                         Vinyl Chloride                                                                            12 12 24 20 30 12 12 12 12                                        Copolymer                                                                     Polyurethane Resin                                                                        5  5  11 15 20 5  5  5  5                                         Isocyanate  1  5  12 15 15 1  5  5  5                                         Density (g/cc)                                                                            2.2                                                                              2.198                                                                            2.201                                                                            2.155                                                                            2.174                                                                            2.195                                                                            2.198                                                                            2.198                                                                            2.198                                     TiO.sub.2 Volume Ratio (%)                                                                35.51                                                                            42.90                                                                            35.65                                                                            30.79                                                                            28.23                                                                            33.22                                                                            42.90                                                                            42.90                                                                            42.90                                     Carbon Black Volume                                                                       20.05                                                                            0.00                                                                             0.00                                                                             7.72                                                                             7.08                                                                             25.00                                                                            0.00                                                                             0.00                                                                             0.00                                      Ratio (%)                                                                     Total Powder Volume                                                                       55.56                                                                            42.90                                                                            35.65                                                                            38.51                                                                            35.32                                                                            58.22                                                                            42.90                                                                            42.90                                                                            42.90                                     Ratio (%)                                                                     __________________________________________________________________________

                                      TABLE 15                                    __________________________________________________________________________              Sample No.                                                                    8-1 8-2                                                                              8-3                                                                              8-4                                                                              8'-1                                                                             8'-2                                                                             8-5                                                                              8'-3                                                                              8-6                                       __________________________________________________________________________    Difference in Powder                                                                    20.81                                                                             8.14                                                                             0.04                                                                             -3.04                                                                            -6.23                                                                            23.47                                                                            8.14                                                                             8.14                                                                              8.14                                      Volume Ratio* (%)                                                             Δd (μm)                                                                        0.18                                                                              0.11                                                                             0.16                                                                             0.16                                                                             0.09                                                                             0.12                                                                             0.22                                                                             0.21                                                                              0.09                                      Evaluation:                                                                   Percent Thermal                                                                         0.21                                                                              0.25                                                                             0.32                                                                             0.36                                                                             0.45                                                                             0.21                                                                             0.3                                                                              0.25                                                                              0.2                                       Shrinkage (%)                                                                 Skewness  22.05                                                                             26.25                                                                            33.6                                                                             37.8                                                                             47.25                                                                            22.1                                                                             31 26.4                                                                              21                                        7 MHZ Output (dB)                                                                       6.7 6.6                                                                              6.2                                                                              6  2.8                                                                              3.6                                                                              5.8                                                                              1.5 6.3                                       C/N (dB)  2.1 2  1.8                                                                              1.8                                                                              1.7                                                                              2.2                                                                              1.6                                                                              1   2                                         Running Durability                                                                      good                                                                              good                                                                             good                                                                             good                                                                             good                                                                             bad                                                                              good                                                                             medium                                                                            good                                                to                        to                                                  medium                    medium                                    __________________________________________________________________________     Note: *(Total powder ratio of layer (a)) - (total powder ratio of layer       (b))                                                                     

The results in Table 15 reveal that each of the samples according to thepresent invention has a smaller skewness, a smaller σ, and a smaller.sup.Δ d as compared with the comparative samples and therefore exhibitssatisfactory reproduction output, C/N, and running durability.Comparative Sample 8'-1 has a small powder volume ratio in layer (a),giving a small powder volume ratio difference (-6.23%). As a result, ithas a high percent thermal shrinkage (0.45%), a large skewness, and alarge σ. On the contrary, Comparative Sample 8'-2 has a very high powdervolume ratio in layer (a), giving a large powder volume ratio difference(23.47%). It has a large a and a large .sup.Δ d and is thereforeinferior in running durability, though showing an improvement onskewness. Comparative Sample 8'-3 is inferior in electromagneticcharacteristics due to the large thickness of layer (b) thereof (1.2μm).

It has now been proved that control of percent thermal shrinkage of amagnetic recording medium at 70° C.×48 hrs is effective to reduceskewness while assuring satisfactory running durability, C/N, andreproduction output. A coated type magnetic recording medium having anextremely thin magnetic layer and still exhibiting such excellentcharacteristics can be produced in large quantity without beingaccompanied with coating defects.

EXAMPLE 9

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Rutile TiO.sub.2 (average particle size:                                                               80     parts                                         0.035 μm; TiO.sub.2 content: ≧90%; BET                              specific surface area: 40 m.sup.2 /g;                                         DBP absorption: 27 to 38 g/100 g;                                             pH: 7)                                                                        Carbon black (average particle size:                                                                   20     parts                                         16 mμ; DBP absorption: 80 ml/100 g;                                        pH: 8.0; BET specific surface area:                                           250 m.sup.2 /g; volatile content: 1.5%)                                       Vinyl chloride-vinyl acetate-vinyl                                                                     12     parts                                         alcohol copolymer (86:13:1; -N(CH.sub.3).sub.3.sup.+ Cl.sup.-                 content: 5 × 10.sup.-6 eq/g; polymerization                             degree: 400)                                                                  Polyester polyurethane resin (neopentyl-                                                               5      parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               Butyl stearate           1      part                                          Stearic acid             1      part                                          Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were kneaded in a continuous kneader and dispersedin a sand mill. To the dispersion were added 1 part of polyisocyanateand 40 parts of butyl acetate, followed by filtration through a filterhaving an average pore size of 1 μm to prepare a coating composition forlayer (a).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: Fe/Zn/Ni alloy                                                                   100    parts                                         (92/4/4; Hc: 1600 Oe; BET specific                                            surface area: 60 m.sup.2 /g; crystallite                                      size: 195 Å; average major axis                                           length: 0.20 μm; acicular ratio:                                           10; σ.sub.s : 130 emu/g)                                                Vinyl chloride copolymer (-SO.sub.3 Na content:                                                        12     parts                                         1 × 10.sup.-4 eq/g; polymerization degree: 300)                         Polyester polyurethane resin (neopentyl-                                                               3      parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               α-Alumina (average particle size: 0.3 μm)                                                     2      parts                                         Carbon black (average particle size:                                                                   0.5    pats                                          0.10 mμ)                                                                   Butyl stearate           1      part                                          Stearic acid             2      parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were kneaded in a continuous kneader and dispersedin a sand mill. To the dispersion were added 3 parts of polyisocyanateand 40 parts of butyl acetate, followed by filtration through a filterhaving an average pore size of 1 μm to prepare a coating composition forlayer (b).

The coating composition for layer (a) was coated on a 7 μm thickpolyethylene terephthalate film support having a centerline averagesurface roughness of 0.01 μm to a dry thickness of 2 μm. Immediatelythereafter while layer (a) was wet, the coating composition for layer(b) was coated thereon to a dry thickness of 0.5 μm (successivewet-on-wet coating). While layers (a) and (b) were wet, theferromagnetic powder in layer (b) was orientated by applying a magneticfield using a cobalt magnet having a magnetic force of 3000 G and asolenoid having a magnetic force of 1500 G. After drying, the coatedfilm was calendered through 7 stages of metallic rolls at 90° C. and cutto a width of 8 mm to prepare a 8 mm-video tape sample. The resultingsample was designated 9-1.

Samples 9-2 to 9-7 and Comparative Samples 9'-1 to 9'-4 were prepared inthe same manner as for Sample 9-1, except for making alterations asshown in Tables 16 and 17 below.

Sample 9-8 was prepared in the same manner as for Sample 9-1, except forusing the following composition for layer (b).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: Ba ferrite (Hc:                                                                  100    parts                                         Oe; BET specific surface area:                                                m.sup.2 /g; crystallite size: Å; average                                  plate diameter: μm; aspect ratio: ;                                        σ.sub.s : emu/g)                                                        Vinyl chloride copolymer (-SO.sub.3 Na content:                                                        12     parts                                         1 × 10.sup.-4 eq/g; polymerization degree: 300)                         Polyester polyurethane resin (neopentyl-                                                               3      parts                                         glycol/caprolactone polyol/MDI = 0.9/2.6/1;                                   -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               α-Alumina (average particle size: 0.3 μm)                                                     5      parts                                         Carbon black (average particle size:                                                                   0.5    pats                                          0.10 mμ)                                                                   Butyl stearate           1      part                                          Stearic acid             2      parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

Each of the resulting samples was evaluated according to the followingtest methods, and the results obtained are shown in Tables 16 and 17.

SMD, STD, Stiffness Ratio (SMD/STD)

A 8 mm wide and 50 mm long specimen punched out of a sample tape wasmade into a loop. A force required for causing a displacement of 5 mminto the inner diameter direction at a rate of displacement of 5 mm/secwas measured for each of machine direction and transverse directions bythe use of a loop stiffness tester manufactured by Toyo Seiki K. K. Fromthe thus obtained SMD and STD values is obtained a stiffness ratio,SMD/STD.

Envelope Flatness

7 MHz signals were recorded on a tape sample by using a 8-mm video deck"FUJI X8". On reproduction of the signals, the output signals wereobserved with an oscilloscope. An envelope flatness is a differencebetween the maximum output and the minimum output in one field.

7 MHz Output

p7 MHz signals were recorded on a tape sample by using a 8 mm video deck"FUJI X8". On reproduction of the signals, the reproduction outputs of 7MHz signals were observed with an oscilloscope. A 8-mm tape "SAG P6-120"produced by Fuji Photo Film Co., Ltd. was used as a reference sample.

C/N

7 MHz signals were recorded on a sample tape using a 8-mm video deck"FUJI X8". The noise generated at 6 MHz on reproduction of the recordedsignals was measured with a spectrum analyzer, and a ratio of thereproduction output to the noise was obtained.

Running Durability

A sample tape in a cassette P6-120 was played 100 passes on ten 8mm-video decks "FUJI X8" in an atmosphere of 23° C. and 70% RH. Areduction in output during 100 passes was measured.

Running durability was evaluated according to the following ratingsystem:

Good . . . Output reduction within 2 dB

Medium . . . Output reduction of from 2 to 4 dB

Bad . . . Output reduction of more than 4 dB or occurrence of jamming

Pinholes

A magnetic layer of a sample before formation of a back layer wasobserved with transmitted white light with naked eye to count the numberof pinholes per 100 m². A satisfactory level is 1 per 100 m².

                                      TABLE 16                                    __________________________________________________________________________                   Sample No.                                                                    9-1 9-2 9'-1                                                                              9-3 9'-2                                                                              9-4                                        __________________________________________________________________________    Layer (a) Inorganic Powder:                                                   Kind           TiO.sub.2                                                                         silica                                                                            carbon                                                                            TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                  Mohs Hardness  7.  6   2   7   7   7                                          Average Particle Size (μm)                                                                0.035                                                                             0.05                                                                              0.02                                                                              0.07                                                                              0.1 0.035                                      Layer (a) Thickness (μm)                                                                  2.5 2.5 2.5 2.5 2.5 2                                          Layer (b) Ferromagnetic Powder                                                               Fe--Ni                                                                            Fe--Ni                                                                            Fe--Ni                                                                            Fe--Ni                                                                            Fe--Ni                                                                            Fe--Ni                                     Layer (b) Thickness d (μm)                                                                0.5 0.5 0.5 0.5 0.5 1                                          σ (μm)                                                                              0.08                                                                              0.11                                                                              0.25                                                                              0.09                                                                              0.21                                                                              0.15                                       Δd (μm)                                                                             0.16                                                                              0.16                                                                              0.27                                                                              0.13                                                                              0.28                                                                              0.35                                       Support Thickness (μm)                                                                    10  10  10  10  10  10                                         Total Tape Thickness (μm)                                                                 13.5                                                                              13.5                                                                              13.5                                                                              13.5                                                                              13.5                                                                              13.5                                       Coating System successive wet-on-wet coating                                  Evaluation:                                                                   STD (mg)       82  75  45  80  65  83                                         SMD (mg)       110 105 90  105 125 111                                        Stiffness Ratio                                                                              1.34                                                                              1.40                                                                              1.96                                                                              1.31                                                                              1.92                                                                              1.34                                       Envelope Flatness (dB)                                                                       -0.3                                                                              -0.5                                                                              -2.5                                                                              -0.2                                                                              -2.2                                                                              -0.1                                       7 MHZ Output (dB)                                                                            6.5 6.4 1.2 6.3 2.1 6.6                                        C/N (dB)       7.8 6.9 2.1 7.5 3.5 7.7                                        Running Durability                                                                           good                                                                              good                                                                              bad medium                                                                            good                                                                              good                                       Pinholes (/100 m.sup.2)                                                                      0   0   0   0   0   0                                          __________________________________________________________________________

                                      TABLE 17                                    __________________________________________________________________________                  9-5   9'-3                                                                              9-4   9-5   9-7                                       __________________________________________________________________________    Layer (a) Inorganic Powder:                                                   Kind          TiO.sub.2 TiO.sub.2                                                                           α-Al.sub.2 O.sub.3                                                            TiO.sub.2                                 Mohs Hardness 7         7     9     7                                         Average Particle Size (μm)                                                               0.035     0.035 0.05  0.035                                     Layer (a) Thickness (μm)                                                                 2.8       1.8   2.5   2.8                                       Layer (b) Ferromagnetic Powder                                                              Fe--Ni                                                                              Fe--Ni                                                                            Fe--Ni                                                                              Fe--Ni                                                                              Ba                                        Layer (b) Thickness d (μm)                                                               0.2       1.2   0.5   0.2                                       σ (μm)                                                                             0.10  --  0.12  0.08  0.09                                      Δd (μm)                                                                            0.06  --  0.25  0.22  0.05                                      Support Thickness (μm)                                                                   10    12.5                                                                              10    10    10                                        Total Tape Thickness (μm)                                                                13.5  13  13.5  13.5  13.5                                      Coating System                                                                              successive                                                                          single                                                                            successive                                                                          successive                                                                          successive                                              wet-on-wet                                                                          layer                                                                             wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                Evaluation:                                                                   STD (mg)      81    failure                                                                           83    92    90                                        SMD (mg)      106   of  110   115   95                                        Stiffness Ratio                                                                             1.31  sample                                                                            1.33  1.25  1.06                                      Envelope Flatness (dB)                                                                      -0.1  prepar-                                                                           -0.1  -0.1  -0.5                                      7 NHz Output (dB)                                                                           7.1   ation                                                                             1.5   7.1   5.5                                       C/N (dB)      8.5       1.2   8     6.8                                       Running Durability                                                                          good      good  good  good                                      Pinholes (/100 m.sup.2)                                                                     0     550 0     0     0                                         __________________________________________________________________________

As can be seen from Tables 16 and 17, the samples according to thepresent invention having its SMD/STD ratio controlled within a range offrom 1.0 to 1.9 exhibit improved contact with a head and reducedenvelope flatness. It also seen that these samples are free from coatingdefects and have a small σ so that they are excellent in runningdurability, output, and C/N ratio. Using carbon having a Mohs hardnessof 2 in layer (a), Comparative Sample 9'-1 has a low STD, failing toobtain a prescribed SMD/STD ratio and to improve envelope flatness, σ,and running durability. Using an inorganic powder having a large averageparticle size, Comparative Sample 9'-2 shows no improvement in envelopeflatness or σ. In the preparation of Comparative Sample 9'-3, which isan example of using no lower layer, the magnetic layer suffered fromcoating defects, giving no sample for evaluation. Since ComparativeExample 9'-4 has a thick magnetic layer (1.2 μm), it achievedimprovements in envelope flatness and running durability, thoughinferior in electromagnetic characteristics.

EXAMPLE 10

A polyethylene terephthalate film support (thickness: 10 μm; FS value:20 kg/mm² in MD, 14 kg/mm² in TD; Young's modulus: 750 kg/mm² in MD, 470kg/mm² in TD) (hereinafter abbreviated as PET support) or a polyethyleneterenaphthalate film support (thickness: 7 μm; F5 value: 22 kg/mm² inMD, 18 kg/mm² in TD; Young's modulus: 750 kg/mm² in MD, 750 kg/mm² inTD) (hereinafter abbreviated as PEN support) was used.

Coating Composition for Subbing Layer

    ______________________________________                                        --SO.sub.3 Na-Containing polyester resin                                                           100       parts                                          (Tg: 65° C.; Na content: 4600 ppm)                                     Cyclohexanone        9900      parts                                          ______________________________________                                    

The above composition was stirred in a disper stirrer for 12 hours, andthe resulting coating composition was coated on the PET or PEN supportwith a bar coater to a dry thickness of 0.1 μM.

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Rutile TiO.sub.2 (average particle size:                                                               85     parts                                         0.035 μm; TiO.sub.2 content: ≧90%; surface                          treating agent: Al.sub.2 O.sub.3 ; BET specific                               surface area: 35 to 45 m.sup.2 /g; true                                       specific gravity: 4.1; pH: 6.5 to 8.0)                                        Carbon black (average particle size:                                                                   5      parts                                         16 mμ; DBP absorption: 80 ml/100 g;                                        pH: 8.0; BET specific surface area:                                           250 m.sup.2 /g; coloring power: 143%)                                         Vinyl chloride copolymer (-SO.sub.3 Na                                                                 13     parts                                         content: 8 × 10.sup.-5 eq/g; containing -OH                             and epoxy groups; Tg: 71° C.; polymer-                                 ization degree: 300; Mn: 12000; Mw:                                           38000)                                                                        Polyurethane resin (-SO.sub.3 Na content:                                                              5      parts                                         8 × 10.sup.-5 eq/g; -OH content: 8 ×10.sup.-5                     eq/g; Tg: 38° C.; Mw: 50000)                                           Cyclohexane              100    parts                                         Methyl ethyl ketone      100    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 4hours. To the mixture were added 5 parts of polyisocyanate (Coronate L),1 part of oleic acid, 1 part of stearic acid, and 1.5 part of butylstearate to prepare a coating composition for layer (a).

    ______________________________________                                        Coating Composition of Layer (b):                                             ______________________________________                                        Ferromagnetic powder: Fe/Co/Ni alloy                                                                   100    parts                                         (92:6:2; sintering inhibitor: Al.sub.2 O.sub.3 ;                              Hc: 1600 Oe; σ.sub.s : 119 emu/g; major                                 axis length: 0.13 μm; acicular ratio:                                      7; crystallite size: 172 Å; water                                         content: 0.6%)                                                                Vinyl chloride copolymer (-SO.sub.3 Na                                                                 13     parts                                         content: 8 × 10.sup.-5 eq/g; containing                                 -OH and epoxy group; Tg: 71° C.; poly-                                 merization degree: 300; number average                                        molecular weight (Mn): 12000; weight                                          average molecular weight (Mw): 38000)                                         Polyurethane resin (-SO.sub.3 Na content:                                                              5      parts                                         8 × 10.sup.-5 eq/g; -OH content: 8 × 10.sup.-5                    eq/g; Tg: 38° C.; Mw: 50000)                                           α-Alumina (average particle size:                                                                12     parts                                         0.15 μm; BET specific surface area:                                        8.7 m.sup.2 /g; pH: 8.2; water content:                                       0.06%)                                                                        Cyclohexanone            150    parts                                         Methyl ethyl ketone      150    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 6hours. To the mixture were added 5 parts of polyisocyanate (Coronate L),1 part of oleic acid, 1 part of stearic acid, and 1.5 part of butylstearate to prepare a coating composition for layer (b).

On the subbing layer side of the support was simultaneously coated withthe coating compositions for layers (a) and (b) by use of two doctorblades set at different gaps to form layer (a) having a dry thickness of3.0 μm and layer (b) having a dry thickness of 0.3 μm. The ferromagneticpowder in layer (b) was orientated with a permanent magnet of 3500 G anda solenoid of 1600 G. After drying, the coated film was supercalenderedthrough metallic rolls at 80° C. and cut to a width of 8 mm to prepare a8 mm-video tape sample.

    ______________________________________                                        Coating Composition for Back Layer:                                           ______________________________________                                        Carbon black (BET specific surface                                                                     100    parts                                         area: 220 m.sup.2 /g; average particle                                        size: 17 mμ; DBP absorption: 75 ml/100 g;                                  volatile content: 1.5%; pH: 8.0.; bulk                                        density: 15 lbs/ft.sup.3)                                                     Nitrocellulose "RS1/2"   100    parts                                         Polyester polyurethane "Nippollan"                                                                     30     parts                                         (produced by Nippon Polyurethane                                              Co., Ltd.)                                                                    Dispersing agent:                                                               Copper oleate          10     parts                                           Copper phthalocyanine  10     parts                                           Barium sulfate (precipitated)                                                                        5      parts                                         Methyl ethyl ketone      500    parts                                         Toluene                  500    parts                                         ______________________________________                                    

The above components were preliminarily kneaded and then kneaded in aroll mill. To 100 parts of the resulting dispersion were added 100 partsof carbon black (BET specific surface area: 200 m² /g; average particlesize: 200 mμ; DBP absorption: 36 ml/100 g; pH: 8.5) and 0.1 part ofα-Al₂ O₃ (average particle size: 0.2 μm), and the mixture was dispersedin a sand grinder, followed by filtration. To 100 parts of the resultingdispersion were further added 120 parts of methyl ethyl ketone and 5parts of polyisocyanate to prepare a coating composition for a backlayer.

The resulting coating composition was coated on the non-magnetic supporton the side opposite to layer (b) with a bar coater to a dry thicknessof 0.5 μm. The resulting magnetic recording medium was cut to a width of8 mm to prepare a 8-mm video tape. The sample using a PET support wasdesignated sample 10-1, and that using a PEN support Sample 10-2.

Each of Samples 10-1 and 10-2 was evaluated according to the followingtest methods.

1) TEM Observations

A sample was sliced with a diamond cutter to prepare an about 0.1 μmthick specimen. The specimen was photographed with TEM. The interfacebetween layers (a) and (b) and the surface of layer (b) were marked, andthe thickness of layer (b) was measured with an image analyzer "IBAS II"to obtain an average d and a standard deviation σ.

As a result, layer (b) had an average dry thickness d of 0.45 μm. Alayer (b) thickness suitable for practical use was proved to be not morethan 1 μm, and particularly not more than 0.6 μm. The standard deviationa of the layer (b) thickness variation was found to be not more than0.08 μm. A practically useful σ was proved to be not more than 0.2 μm,and particularly not more than 0.1 σm. The same measurements on Sample2-4 gave d=0.28 μm, and σ=0.06 μm.

The above-prepared sample was stretched to have layer (b) released fromthe support, and layer (b) was scraped off with a cutter blade. The thusremoved layer (b) weighing 500 mg was refluxed in 100 ml of a 1NNaOH/methanol solution for 2 hours to hydrolyze the binders. Thesupernatant liquor was removed with the ferromagnetic powder of greaterspecific gravity being precipitated. The solid was washed three timeswith water and then three times with tetrahydrofuran, followed by dryingin a vacuum drier at 50° C. The resulting ferromagnetic powder wasdispersed in collodion and observed under TEM (×60000). Theferromagnetic powder was found to have a major axis length of 0.13 μmand an acicular ratio of 10. The same measurements revealed that theferromagnetic-powder used in Sample 2-2 had a major axis length of 0.25μm and an acicular ratio of 15. It was proved that a major axis lengthshould be not more than 0.4 μm, and preferably not more than 0.3 μm, forpractical use and that an acicular ratio should fall within a range offrom 2 to 20, and preferably from 2 to 15, for practical use.

2) Atomic Force Microscope (AFM)

Surface roughness R_(rms) was obtained by scanning the surface of layer(b) with "Nanoscope II" manufactured by Digital Instrument Co. over anarea of 6 μm×6 μm at a tunnel current of 10 nA and a bias voltage of 400mV.

As a result, R_(rms) was 6 nm. It was proved that R_(rms), should no notmore than 20 nm, and preferably not more than 10 nm, for practical use.The data of Example 2 can also be referred to.

3) Surface Roughness Tester

Surface roughness was measured using 3d-MIRAU. Centerline surfaceroughness (Ra), R_(rms) and peak-to-valley value of layer (b) over anarea of about 250×250 mm were measured according to a MIRAU method bymeans of "TOPO 3D" manufactured by WYKO K.K. Spherical corrections andcylindrical corrections were made at a measuring wavelength of about 650nm. This testing system is a non-contact roughness tester utilizinginterference of light.

As a result, Ra was 2.7 nm. It was proved that a practically useful Rais from 1 to 4 nm, and particularly from 2 to 3.5 nm. R_(rms) was 3.5nm. A practically useful R_(rms) is from 1.3 to 6 nm, and particularlyfrom 1.5 to 5 nm. P-V value was between 20 and 30 nm. A practicallyuseful P-V value is not more than 80 nm, and particularly from 10 to 60nm.

4) Vibrating Sample Magnetometer

Magnetic characteristics of the sample were measured at Hm of 5 kOe withVSM manufactured by Toei Kogyo K.K.

The result were Hc: 1620 Oe; Hr (90°): 1800 Oe; Br/Bm: 0.82; SED: 0.583.For practical use, it was proved that Hc should be from 1500 to 2500 Oe,and preferably from 1600 to 2000 Oe; Hr (90°) should be from 1000 to2800 Oe, and preferably from 1200 to 2500 Oe; Br/Bm should be at least0.75, and preferably at least 0.8; and SFD should be 0.7 or less, andpreferably 0.6 or less.

The same measurements on the samples prepared in Example 4 gave similarresults.

5) X-Ray Diffraction

X-ray diffratometry was conducted using the ferromagnetic powder sampledfrom layer (b) in (1) above. The magnetic tape was directly set on anX-ray diffractometer. A crystallite size of the ferromagnetic powder wasobtained from the half-width value of the diffraction pattern of thefaces (1,1,0) and (2,2,0). As a result, the crystallite size was 180 Å.For practical use, a crystallite size is preferably not more than 400 Å,and more preferably from 100 to 300 Å. The same measurements on Sample6-2 gave 280 Å.

6) Tensile Test

Young's modulus, yield stress, and yield elongation of the magnetic tapewere measured with a tensile tester "STM-T-50BP" manufactured by ToyoBaldwin K.K. in an atmosphere of 23° C. and 70% RH at a rate of pullingof 10%/min.

As a result, the sample had a Young's modulus of 700 kg/mm², a yieldstress of from 6 to 7 kg/mm², and a yield elongation of 0.8%. Forpractical use, a Young's modulus preferably ranges from 400 to 2000kg/mm², and more preferably from 500 to 1500 kg/mm² ; a yield stresspreferably ranges from 3 to 20 kg/mm², and more preferably from 4 to 15kg/mm² ; a yield elongation preferably ranges from 0.2 to 8%, and morepreferably from 0.4 to 5%.

7) Stiffness in Flexure, Loop Stiffness

Stiffness in flexure was expressed in terms of a force (mg) required forgiving a 5 mm displacement to a 8 mm wide and 50 mm long sample in aloop form with use of a loop stiffness tester at a rate of displacementof about 3.5 mm/sec.

As a result, the 8 mm wide p6-120 tape having a thickness of 10.5. μmhad a stiffness between 40 and 60 mg. With a thickness of 10.5±1 μm, apreferred stiffness is from 20 to 90 mg, and particularly from 30 to 70mg, for practical use. With a thickness of 11.5 μm or more, a preferredstiffness is from 40 to 200 mg. With a thickness of 9.5 μm or less, apreferred stiffness is from 10 to 70 mg.

8) Elongation at Failure

Elongation at cracking was measured at 23° C. and 70% RH. A 10 cm longspecimen was pulled at both ends thereof at a rate of 0.1 mm/sec whilemicroscopically observing the surface of layer (b). The elongation (%)at which 5 or more clear cracks developed on the surface of layer (b)was measured.

As a result, the sample had an elongation at cracking of 4%. The samemeasurement on Sample 8-4 gave a result of 12%. It was proved that apreferred elongation at cracking is not more than 20%, and particularlynot more than 10%, for practical use.

9) ESCA

Cl/Fe spectrum α and N/Fe spectrum β were measured with an X-rayphotoelectric spectrophotometer (manufactured by Perkin-Elmer Co.) at300 W using an Mg anode as an X-ray source. After washing away thelubricant in the sample with n-hexane, the sample was set in an X-rayphotoelectric spectrophotometer at a distance of 1 cm from the X-raysource. After 5 minutes from evacuation, Cl-2P spectrum, N-1S spectrum,and Fe-2P (3/2) spectrum were integrated for 10 minutes. A pass energywas fixed at 100 eV. An integrated intensity ratio of the Cl-2P spectrumto the Fe-2P (3/2) spectrum was calculated to obtain α. An integratedintensity ratio of the N-1S spectrum to the Fe-2P (3/2) spectrum wascalculated to obtain β.

As a result, α was 0.45, and β was 0.07. The same measurements on Sample3-5 gave α of 0.32 and β of 0.10. It was proved that a practicallypreferred range of α is from 0.3 to 0.6, and particularly from 0.4 to0.5, and that of β is from 0.03 to 0.12, and particularly from 0.04 to0.1.

10) Dynamic Viscoelastometer

Dynamic viscoelasticity of the sample was measured at 110 Hz with adynamic viscoelastometer "Rheovibron" manufactured by Toyo Baldwin Co.The peak temperature at E" was taken as Tg. This measurement systemcomprises adding vibration to one end of the tape and measuring thevibration transmitted to the other end.

It was found, as a result, that Tg was 73° C.; E' (50° C.) was 4×10¹⁰dyne/cm² ; and E" (50° C.) was 1×10¹¹. It was proved that a practicallypreferred range of Tg is from 40 to 120° C., and particularly from 50 to110° C., that of E' (50° C.) is from 0.8×10¹¹ to 11×10¹¹ dyne/cm², andparticularly from 1×10¹¹ to 9×10¹¹ dyne/cm², and that of E" (50° C.) isfrom 0.5×10¹¹ to 8×10¹¹ dyne/cm², and particularly from 0.7×10¹¹ to5×10¹¹ dyne/cm².

11) Adhesive Strength

Adhesive tape produced by 3M was adhered onto a 8 mm wide sample, and a180° peel strength between the support and the magnetic layer wasmeasured at 23° C. and 70% RH.

As a result, the adhesive strength was 50 g. The same measurement onSample 3-1 gave an adhesive strength of 25 g. It was proved that anadhesive strength is preferably 10 g or more, and particularly 20 g ormore, for practical use.

12) Wearability

The sample was placed on a slide glass with both ends thereof fixed withadhesive tape, and a steel ball 6.25 mm in diameter was slid thereonunder a load of 50 g. In this case, the ball was once slid over adistance of 20 mm at a speed of 20 mm/sec and then moved to a freshmagnetic layer surface, where the same sliding was repeated 20 times.Thereafter, the sliding surface of the steel ball was observed with amicroscope (×40) to obtain its diameter, assuming the sliding surfacebeing a circle. The abrasion wear was calculated from the measureddiameter.

As a result, the abrasion wear was found to be 0.7×10⁻⁵ to 1.1×10⁻⁵ mm³.The result of the same measurement on Sample 3-2 was 4×10⁻⁵ mm³. Forpractical use, the abrasion wear was from 0.1×10⁻⁵ to 5×10⁻⁵ mm³, andparticularly from 0.4×10⁻⁵ to 2×10⁻⁵ mm³.

13) Scanning Electron Microscope (SEM)

Five micrographs were taken of the surface of layer (b) with SEM "S-900"manufactured by Hitachi, Ltd. (×5000). The average number of abrasivegrains was found to be 0.2/μm². That of Sample 4-6 was found to be0.4/μm². It was proved that a practically usually number of abrasivegrains is at least 0.1/μm², and particularly from 0.12 to 0.5/μm².

14) Gas Chromatography (GC)

A specimen having an area of 20 cm² was heated to 120° C., and theresidual solvent was measured with a gas chromatograph "GC-14A"manufactured by Shimazu Seisakusho Ltd.

As a result, the residual solvent was 8 mg/m². The result of the samemeasurement on Sample 1-1 was 18 mg/m². It was found that a residualsolvent is preferably not more than 50 mg/m², and particularly not morethan 20 mg/m², for practical use.

15) Sol Fraction

A weight ratio of tetrahydrofuran-soluble solid contents of the magneticlayer to the magnetic layer was found to be 7%. That of Sample 1-1 was5%. It was proved that a sol fraction is preferably not more than 15%,and particularly not more than 10%, for practical use.

16) Magnetic Development Pattern

Short wave recording at 1 MHz was conducted on the 8 mm magneticrecording tape by using a video tape recorder "EVO-9500" produced bySony Corporation. The recorded tape was trimmed to obtain only therecorded portion measuring 5 mm in width and subjected to magneticdevelopment by treating with a solution of Ferricolloid (produced byTaiho Kogyo K.K.; particle size: about 100 Å) and then soaked in aligroin solution for 24 hours. The thus treated tape was photographedwith a differential interference microscope produced by Nippon KogakuK.K. at a magnification of 10 with blue interference color. Visualobservation of the micrographs revealed that no black or white lineappears on those samples whose magnetic layer has an even thickness, butan increase in variation of the magnetic layer thickness is attended byappearance of black or white lines. These lines correspond to theportion suffering from unevenness of thickness. It is desirable that thenumber of such black and white lines within 5 mm width should be 5 orless. Further, the difference in density between the white lines andblack lines as measured with a microdensitometer is preferably not morethan 0.2, and more preferably not more than 0.1.

17) Coefficient of Friction (μ)

The 8 mm tape was run in contact with a rod of SUS 420J (diameter: 4 mm)under a tension of 20 g (T1) at a lap angle of about 180° C. The tension(T2) necessary for running the tape at a speed of 14 mm/sec wasmeasured. The coefficient of friction (μ) was obtained according toequation:

    μ=(1/π)·1n(T1/T2)

As a result, μ on the magnetic surface was 0.3. For practical use, μ onthe magnetic surface preferably ranges from 0.15 to 0.4, and morepreferably from 0.2 to 0.35. μ on the back layer was 0.2. It was provedthat μ on the back side is preferably from 0.15 to 0.4, and moreparticularly from 0.2 to 0.35, for practical use.

The coefficient of friction is chiefly influenced by the magneticsubstance, abrasive, carbon black, lubricants, and dispersing agentsused.

18) Contact Angle

A drop of water or methylene iodide was put on the magnetic layer, andthe contact angle was measured with a microscope.

The contact angle with water was 90°, and that with methylene iodide was20°. For practical use, it was proved that a contact angle with water ispreferably from 60° to 130°, and particularly from 80° to 120°, and thatwith methylene chloride is preferably from 10° to 90°, and particularlyfrom 10° to 70°.

These contact angles are decided particularly by the lubricating agentor dispersing agent used.

19) Surface Free Energy

Surface free energy of the magnetic layer and the back layer wasmeasured according to the method described in JP-A-3-119531, D. K.Owens, J. Appl. Polymer Sci., Vol. 13 (1969), and J. Panzer, J. Colloid& Interfacial Sci., Vol. 44, No. 1.

As a result, the surface free energies of the magnetic layer and theback layer were both 40 dyne/cm. It was proved that a practicallypreferred surface free energy was from 10 to 100 dyne/cm.

The surface free energy is decided particularly by lubricating agents ordispersing agents used.

20) Surface Resistivity

A 8 mm wide specimen was set over a pair of electrodes having a crosssection of a quarter of a circle 10 mm in radius, placed 8 mm apart, andsurface resistivity was measured with a digital surface resistivitymeter "TR-8611 A" manufactured by Takeda Riken K.K.

The surface resistivity both of the magnetic layer and the back layerwas 1×10⁶ Ω/sq. It was found that the surface resistivity is preferablynot more than 1×10⁹ Ω/sq, and particularly not more than 1×10⁸ Ω/sq, forpractical use.

The surface resistivity is decided by ferromagnetic powders, binders,carbon black, etc. used.

Samples 10-1 and 10-2 were compared with commercially available 8-mmvideo tapes according to the above-mentioned test methods or commonlyemployed methods, and the results obtained are shown in Table 18.Standards for rating the results were as follows.

Jitter

Good . . . less than 0.2 μsec

Bad . . . 0.2 μsec or more

Preservation Stability

Good . . . No rust occurred after preservation at 60° C. and 90% RH for10 days.

Bad . . . Rust occurred after preservation at 60° C. and 90% RH for 10days.

Running Durability

Good . . . No jamming lasting 30 seconds or longer occurred during 50passes on a 8-mm video deck.

Bad . . . Jamming lasting 30 seconds or longer occurred during 50 passeson a 8-mm video deck.

Scratch Resistance

Good . . . No scratches were visually perceived after 10 minutes runningin a still mode.

Bad . . . Scratches were visually perceived after 10 minutes running ina still mode.

                  TABLE 18                                                        ______________________________________                                                                 Single-                                                       Sample Sample   Coated    Deposited                                           10-1   10-2     Metal Tape.sup.1)                                                                       Tape.sup.2)                                ______________________________________                                        Electromagnetic                                                               Characteristics:                                                              7 MHz Output (dB)                                                                        5.5      6.0      3.0     6.2                                      C/N (dB)   4.3      4.5      2.0     4.1                                      Color S/N (dB)                                                                           2.5      2.6      2.5     -3.0                                     Video S/N (dB)                                                                           2.1      2.3      1.5     0.5                                      Durability:                                                                   Dropout    40       30       30      580                                      BER (×10.sup.-5)                                                                   4        2        50      80                                       Jitter     good     good     good    bad                                      Still      ≧30 min                                                                         ≧30 min                                                                         ≧30 min                                                                        ≧30 min                           Head Wear  1.2      1.4      2.0     0.2                                      (μm/100 hr)                                                                Preservation                                                                             good     good     good    bad                                      Stability                                                                     (60° C., 90% RH)                                                       Running    good     good     good    bad                                      Durability                                                                    Scratch    good     good     good    medium                                   Resistance                           to bad                                   ______________________________________                                         Note:                                                                         .sup.1) : Product of Fuji Photo Film Co., Ltd.; Lot No. 407209M               .sup.2) : Product of Sony Corporation; Lot No. 709011CD                  

EXAMPLES 11

Example 11 relates to means (A).

    ______________________________________                                        Coating Composition for Layer (a-A):                                          ______________________________________                                        Inorganic powder α-Fe.sub.2 O.sub.3 (average                                                    80     parts                                          particle size: 0.27 μm; BET specific                                       surface area: 18 m.sup.2 /g; pH: 5.5)                                         Carbon black (average primary                                                                         20     parts                                          particle diameter: 16 mμ; DBP                                              absorption: 80 ml/100 g; pH: 8.0;                                             BET specific surface area: 250 m.sup.2 /g;                                    volatile content: 1.5%)                                                       Vinyl chloride-vinyl acetate-vinyl                                                                    12     parts                                          alcohol (86:13:1) copolymer (polar                                            group (-N(CH.sub.3).sub.3.sup.+ Cl.sup.-1) content: 5 ×                 10.sup.-6 eq/g; degree of polymerization:                                     400)                                                                          Polyester polyurethane resin                                                                          5      parts                                          (neopentyl glycol/caprolactone                                                polyol/diphenylmethane-4,4'-                                                  diisocyanate (MDI) = 0.9/2.6/1;                                               -SO.sub.3 Na content: 1 × 10.sup.-4 eq/g)                               Butyl stearate          1      part                                           Stearic acid            1      part                                           Copper oleate           1      part                                           Methyl ethyl ketone     200    parts                                          ______________________________________                                    

The above components were kneaded in a continuous kneader and dispersedin a sand mill. Five parts of polyisocyanate were added to thedispersion, and 40 parts of butyl acetate was further added. Thedispersion was filtered through a filter having an average pore size of1 μm to prepare a coating composition for lower non-magnetic layer(a-A). An α-Fe₂ O₃ to carbon black ratio in the composition was 8/2.

Coating Composition for Layer (a-B)

A coating composition for lower non-magnetic layer (a-B) was prepared inthe same manner as for the coating composition for layer (a-A), exceptfor increasing the amount of butyl acetate to 350 parts for dilution.

Coating Compositions for Layers (a-C), (a-D), (a-E), and (a-F)

A coating composition for lower non-magnetic layer (a-C), (a-D) or (a-E)was prepared in the same manner as for layer (a-A), except for changingthe α-Fe₂ O₃ to carbon black ratio to 9/1, 5/5, or 4/6, respectively.

A coating composition for lower non-magnetic layer (a-F) was prepared inthe same manner as for layer (a-A), except for using no carbon black.

Coating Composition for Layer (a-G)

A coating composition for lower non-magnetic layer (a-G) was prepared inthe same manner as for layer (a-A), except for using no polyisocyanate.

Coating Composition for Layer (a-H)

A carbon black dispersion was prepared by kneading the followingcomponents in a continuous kneader and then dispersing in a sandgrinder.

    ______________________________________                                        Carbon black (average primary                                                                          100    parts                                         particle diameter: 16 mμ; DBP                                              absorption: 80 ml/100 g; pH: 8.0;                                             BET specific surface area:                                                    250 m.sup.2 /g; volatile content: 1.5%)                                       Vinyl chloride-vinyl acetate-vinyl                                                                     100    parts                                         alcohol (86:13:1) copolymer (polar                                            group (-N(CH).sub.3.sup.+ Cl.sup.-) content: 5 ×                        10.sup.-6 eq/g; degree of polymeriza-                                         tion: 400)                                                                    Polyester polyurethane resin                                                                           5      parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            Copper oleate            5      parts                                         Phthalocyanine Blue type dispersing agent                                                              5      parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The following components were mixed and then dispersed in a sandgrinder.

    ______________________________________                                        Inorganic powder α-Fe.sub.2 O.sub.3 (average                                                     80     parts                                         particle size: 0.27 μm; BET                                                specific surface area: 18 m.sup.2 /g;                                         pH: 5.5)                                                                      Vinyl chloride-vinyl acetate-vinyl                                                                     5      parts                                         alcohol (86:13:1) copolymer (polar                                            group (-N(CH).sub.3.sup.+ Cl.sup.-) content: 5 ×                        10.sup.-6 eq/g; degree of polymeriza-                                         tion: 400)                                                                    Polyester polyurethane resin                                                                           4      parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            Carbon black dispersion prepared above                                                                 83     parts                                         Butyl stearate           1      part                                          Stearic acid             1      part                                          Copper oleate            1      part                                          Methyl ethyl ketone      200    parts                                         ______________________________________                                    

To the resulting dispersion was added 6 parts of polyisocyanate toprepare a coating composition of lower non-magnetic layer (a-H).

    ______________________________________                                        Coating Composition for Layer (b-A):                                          ______________________________________                                        Ferromagnetic metal fine powder                                                                       100    parts                                          (Fe/Zn/Ni = 92/4/4; Hc: 1600 Oe;                                              BET specific surface area: 60 m.sup.2 /g;                                     crystallite size: 195 Å; particle                                         size (major axis): 0.20 μm;                                                aspect ratio: 10; σ.sub.2 : 130 emu/g)                                  Vinyl chloride copolymer (-SO.sub.3 Na                                                                12     parts                                          content: 1 × 10.sup.-4 eq/g; degree of                                  polymerization: 300)                                                          Polyester polyurethane resin                                                                          3      parts                                          (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            α-Alumina (particle size: 0.3 μm)                                                            2      parts                                          Carbon black (particle size: 0.10 μm)                                                              0.5    part                                           Butyl stearate          1      part                                           Stearic acid            2      parts                                          Methyl ethyl ketone     200    parts                                          ______________________________________                                    

The above components were kneaded in a continuous kneader and thendispersed in a sand mill. To the resulting dispersion was added 6 partsof polyisocyanate, and 40 parts of butyl acetate was further added. Thedispersion was filtered through a filter having an average pore size of1 μm to prepare a coating composition of upper magnetic layer (b-A).

    ______________________________________                                        Coating Composition for Layer (b-B):                                          ______________________________________                                        Hexagonal barium ferrite (Hc:                                                                          100    parts                                         1250 Oe; BET specific surface                                                 area: 47 m.sup.2 /g; particle size:                                           0.05 μm; aspect ratio: 5)                                                  Vinyl chloride-vinyl acetate-vinyl                                                                     13     parts                                         alcohol (86:13:1) copolymer (-SO.sub.3 Na                                     content: 5 × 10.sup.-6 eq/g; degree of                                  polymerization: 400)                                                          Polyester polyurethane resin                                                                           3      parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            α-Alumina (particle size: 0.3 μm)                                                             2      parts                                         Carbon black (particle size: 80 μm)                                                                 0.5    part                                          Butyl stearate           1      part                                          Stearic acid             2      parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were kneaded in a continuous kneader and thendispersed in a sand mill. To the resulting dispersion was added 6 partsof polyisocyanate, and 40 parts of butyl acetate was further added. Thedispersion was filtered through a filter having an average pore size of1 μm to prepare a coating composition of upper magnetic layer (b-B).

Preparation of Sample 11-1

A polyethylene terephthalate film having a thickness of 7 μm and acenterline surface roughness (Ra) of 0.01 μm was coated with the coatingcomposition for layer (a-A) to a dry thickness of 3 μm and immediatelythereafter with the coating composition for layer (b-A) to a drythickness of 0.2 μm by wet-on-wet coating to form layers (a-A) and(b-A). While both layers (a-A) and (b-A) were wet, the ferromagneticpowder in layer (b-A) was orientated by applying a magnetic field usingcobalt magnet having a magnetic force of 3000 G and a solenoid having amagnetic force of 1500 G. After drying, the coated film was calenderedthrough 7 stages of metallic rolls at 90° C. and cut to a width of 8 mmto prepare 8 mm-video tape.

Preparation of Samples 11-2 to 11-7 and Comparative Samples 11'-1 to11'-8

8 mm-Video tape samples were prepared in the same manner as for Sample11-1, except for changing the coating compositions and the thickness oflayers (a) and (b) as shown in Table 19 below. Of these Samples,Comparative Samples 11'-2 and 11'-8 had a single layer structurecomprising layer (b) alone. Comparative Samples 11'-3 and 11'-4 wereprepared by the so-called wet-on-dry coating method as follows. Thecoating composition for layer (a) was coated to a dry thickness of 2.7μm, dried, rolled, and calendered. The thus formed layer (a) wassubjected to a curing treatment at 80° C. for 24 hours. The coatingcomposition for layer (b) was then coated thereon to a dry thickness of0.5 μm, and subjected to orientation, dried, rolled up, and calenderedin the same manner as for Sample 11-1.

Performance properties of each of the resulting samples were evaluatedaccording to the following test methods. The results obtained are shownin Table 19. A A10⁴ /A10 ratio of the coating composition for layer (b)is also shown in Table 19.

Preservation Stability

After being preserved at 60° C. and 90% RH (relative humidity) for 48hours, the sample in a cassette P6-120 was played 10 passes on ten 8mm-video decks "FUJI X8" manufactured by Fuji Photo Film Co., Ltd. in anatmosphere of 23° C. and 70% RH. A reduction in output during 10 passeswas measured. Further, the degree of contamination in the inside of thedeck was observed. Preservation stability was evaluated by rating theresults according to three grades "good", "medium", and "bad".

Electromagnetic Characteristics

1) 7 MHz Output

Signals of 7 MHz were recorded on a sample by means of FUJI X8, and theoutput on reproduction was measured with an oscilloscope.

2) C/N

Signals of 7 MHz were recorded on a sample by means of FUJI X8, and thenoise generated on reproduction at 6 MHz was measured with a spectrumanalyzer to obtain a ratio of reproduced signal to noise.

Contact with Head

Signals of 7 MHz were recorded on a sample by means of FUJI X8, and theenvelope wave form before demodulation in reproduction was observed withan oscilloscope. If the output was assured to show a flat wave form, thecontact with head was judged "good". If a drop of output was observedanywhere, the contact with head was judged "medium" or "bad" accordingto the degree of the output drop.

Yield

The yield of the products was expressed in terms of a percentage ofnon-defectives produced from one jumbo roll on a P6-120 conversion.

                                      TABLE 19                                    __________________________________________________________________________                               Compara.                                                                            Compara.                                                                           Compara.                                                                            Compara.                                        Sample                                                                            Sample                                                                             Sample                                                                            Sample                                                                              Sample                                                                             Sample                                                                              Sample                                          11-1                                                                              11-2 11-3                                                                              11'-1 11'-2                                                                              11'-3 11'-4                             __________________________________________________________________________    Layer (b):                                                                    Coating Composition                                                                         A   A    A   A     A    A     A                                 Thickness (μm)                                                                           0.2 0.5  1   1.3   2    0.5   0.5                               Layer (a):                                                                    Coating Composition                                                                         A   A    A   A     none A     B                                 α-Fe.sub.2 O.sub.3 :Carbon Black Ratio                                                8:2 8:2  8:2 8:2   --   8:2   8:2                               Polyisocyanate (part)                                                                       5   5    5   5     --   5     5                                 Coating System                                                                              wet-on-                                                                           wet-on-                                                                            wet-on-                                                                           wet-on-                                                                             single                                                                             wet-on-                                                                             wet-on-                                         wet wet  wet wet   layer                                                                              dry   dry                               A10.sup.4 (dyne/cm.sup.2)                                                                   3000                                                                              3000 3000                                                                              3000       3000  3000                              A10 (dyne/cm.sup.2)                                                                         300 300  300 300        300   300                               A10.sup.4 /A10                                                                              10  10   10  10         10    10                                Evaluation:                                                                   7 MHZ Output (dB)                                                                           3.5 3    2   0.5   0.2  2.5   -2                                C/N Ratio     3.2 3    2.8 0.3   0.5  0.5   -1.1                              Yield (%)     98  100  98  100   100  3     85                                Preservation Stability                                                                      good                                                                              good good                                                                              good  good bad   bad                               Contact with Head                                                                           good                                                                              good good                                                                              good  good good  good                              __________________________________________________________________________                         Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                               Sample                                                                            Sample                                                                            Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                            Sample                           11-4         11-5                                                                              11'-5                                                                             11'-6                                                                              11'-7                                                                              11'-8                                                                              11-6 11-7                                 __________________________________________________________________________    Layer (b):                                                                    Coating Composition                                                                        A   A   A    A    A.   A    B   A                                Thickness (μm)                                                                          0.5 0.5 0.5  0.5  0.5  0.5  0.5 0.2                              Layer (a):                                                                    Coating Composition                                                                        C   D   E    F    G    none A   H                                α-Fe.sub.2 O.sub.3 :Carbon Black Ratio                                               9:1 5:1 10:0 4:6  8:2  --   8:2 8:2                              Polyisocyanate (part)                                                                      5   5   5    5    o    --   5   5                                Coating System                                                                             wet-on-                                                                           wet-on-                                                                           wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            single                                                                             wet-on-                                                                           wet-on-                                       wet wet wet  wet  wet  layer                                                                              wet wet                              A10.sup.4 (dyne/cm.sup.2)                                                                  6000                                                                              3500                                                                              10000                                                                              4000 3200      3000                                                                              4000                             A10 (dyne/cm.sup.2)                                                                        60  1000                                                                              80   1500 350       300 300                              A10.sup.4 /A10                                                                             100 3.5 125  2.7  9.1       10  13.3                             Evaluation:                                                                   7 MHZ Output (dB)                                                                          3.6 2.6 2.6  0.5  3.5  2.2  3   3.6                              C/N Ratio    3.5 2.5 2.5  0.1  3.2  1.5  3   3.8                              Yield (%)    96  97  1    15   99   1    98  98                               Preservation Stability                                                                     good                                                                              good                                                                              good good bad  good good                                                                              good                             Contact with Head                                                                          good                                                                              good                                                                              bad  good good bad  good                                                                              good                             __________________________________________________________________________

As is apparent from the results in Table 19, any of samples 11-1 through11-7 falling within means (A) of the present invention had a high outputat 7 MHz and a satisfactory C/N and exhibited satisfactory results inall the items of yield, preservation stability and contact with a head.To the contrary, Comparative Sample 11'-1 in which layer (b) has athickness greater than 1 μm had a reduced output at 7 MHz and a reducedC/N. Comparative Sample 11'-2 having a single layer also had a reduced 7MHz output and a reduced C/N. Further, Comparative Samples 11'-3 and11'-4 in which layers (a) and (b) were formed by a wet-on-dry coatingsystem both had a low C/N and unsatisfactory preservation stability.Comparative Samples 11'-5 and 11'-6 whose A10⁴ /A10 ratio is out of therange of the present invention had an extremely poor yield inproduction. Comparative Sample 11'-7 in which layer (a) contained nopolyisocyanate underwent serious deterioration during preservation.

EXAMPLE 12

Means (B) of the present invention was examined. Coating compositionsfor layers (a) and (b) used for sample preparation were prepared asfollows.

    ______________________________________                                        Layer (a):                                                                    Inorganic powder TiO.sub.2 (TTO-55A                                                                    100    parts                                         produced by Ishihara Sangyo Kaisha,                                           Ltd.) (average primary particle                                               diameter: 0.05 μm; BET specific                                            surface area: 18 m.sup.2 /g; pH: 7)                                           Carbon black (average primary                                                                          20     parts                                         particle diameter: 18 mμ; DBP                                              absorption: 80 ml/100 g; pH: 8.0;                                             BET specific surface area: 250 m.sup.2 /g;                                    volatile content: 1.5%)                                                       Vinyl chloride-vinyl acetate-vinyl                                                                     12     parts                                         alcohol (86:13:1) copolymer (polar group                                      (-N(CH.sub.3).sub.3.sup.+ Cl.sup.-1) content: 5 × 10.sup.-6 eq/g;       degree of polymerization: 400)                                                Polyester polyurethane resin                                                                           5      parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            Butyl stearate           1      part                                          Stearic acid             1      part                                          Methyl ethyl ketone      200    parts                                         ______________________________________                                        Layer (b):                                                                    Ferromagnetic metal fine powder                                                                        100    parts                                         (Fe/Zn/Ni = 92/4/4; Hc: 1600 Oe;                                              BET specific surface area: 60 m.sup.2 /g;                                     crystallite size: 195 Å; particle                                         size (major axis): 0.20 μm;                                                aspect ratio: 10; σ.sub.2 : 130 emu/g)                                  Vinyl chloride copolymer (-SO.sub.3 Na                                                                 12     parts                                         content: 1 × 10.sup.-4 eq/g; degree of                                  polymerization: 300)                                                          Polyester polyurethane resin                                                                           3      parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            α-Alumina (particle size: 0.3 μm)                                                             2      parts                                         Carbon black (particle size: 0.10 μm)                                                               0.5    part                                          Butyl stearate           1      part                                          Stearic acid             2      parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components for each of layers (a) and (b) were kneaded in acontinuous kneader and then dispersed in a sand mill. To the resultingdispersions for layer (a) and layer (b) was added 1 part and 3 parts ofpolyisocyanate, respectively, and 40 parts of butyl acetate was furtheradded to each of the dispersions. The dispersion was filtered through afilter having an average pore size of 1 μm to prepare a coatingcomposition for layer (a) and a coating composition for layer (b). TheA10⁴ /A10 ratio of these coating compositions is shown in Table 22below.

Preparation of Sample 12-1

A polyethylene terephthalate film having a thickness of 7 μm and acenterline surface roughness (Ra) of 0.01 μm was coated with the coatingcomposition for layer (a) to a dry thickness of 2 μm and immediatelythereafter with the coating composition for layer (b) to a dry thicknessof 0.3 μm by wet-on-wet coating to form layers (a) and (b). While bothlayers were wet, the ferromagnetic powder in layer (b) was orientated byapplying a magnetic field using cobalt magnet of 3000 G and a solenoidof 1500 G. After drying, the coated film was calendered through 7metallic rolls at 90° C. and cut to a width of 8 mm to prepare 8mm-video tape.

Preparation of Samples 12-2 to 12-5 and Comparative Samples 12'-1 to12'-5

8 mm-Video tape samples were prepared in the same manner as for Sample12-1, except for changing the coating compositions and the thickness oflayers (a) and (b) as shown in Table 20 below. Comparative Sample 12'-2was prepared by so-called wet-on-dry coating as follows. The coatingcomposition for layer (a) was coated to a dry thickness of 2.0 μm,dried, rolled, and calendered. The thus formed layer (a) was subjectedto a curing treatment at 80° C. for 24 hours, and the coatingcomposition for layer (b) was then coated thereon to a dry thickness of0.3 μm, subjected to orientation, dried, rolled up, and calendered inthe same manner as for Sample 12-1.

Performance properties of each of the resulting samples were evaluatedaccording to the following test methods. The results obtained are shownin Table 20.

Running Durability

A sample in a cassette P6-120 was played 100 passes on ten 8 mm-videodecks "FUJI X8" in an atmosphere of 23° C. and 70% RH. A reduction inoutput during 100 passes was measured. Further, any contamination in theinside of the deck was observed. Preservation stability was evaluated byrating the results in five grades "good", "good to medium", "medium","medium to bad", and "bad".

Electromagnetic Characteristics

Evaluated in the same manner as in Example 11.

Yield

A percentage of non-defectives after cutting to a width of 8 mm wasobtained. In general, a yield of at least 97% is demanded.

                                      TABLE 20                                    __________________________________________________________________________                         Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                      Sample                                                                            Sample                                                                            Sample                                                                            Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                              12-1                                                                              12-2                                                                              12-3                                                                              12'-1                                                                              12'-2                                                                              12'-3                                                                              12'-4                                                                              12'-5                                                                              12-4 12-5                       __________________________________________________________________________    Thickness of                                                                           0.3 0.3 0.9 1.1  0.3  0.3  0.3  0.3  0.3  0.3                        Layer (b) (μm)                                                             Layer (a):                                                                    Inorganic Powder:                                                             Kind     TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                         TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          α-Al.sub.2 O.sub.3                                                           SnO.sub.2                  Size* (μm)                                                                          0.05                                                                              0.07                                                                              0.05                                                                              0.05 0.05 0.1  0.05 --   0.06 0.05                       Amount (part)                                                                          100 100 100 100  100  100  100  --   100  100                        Carbon Black:                                                                 Size* (μm)                                                                          --  --  --  --   --   --   --   0.05 --   --                         Amount (part)                                                                          0   0   0   0    0    0    0    100  0    0                          Amount of Poly-                                                                        1   1   1   1    1    1    0    1    1    1                          isocyanate (part)                                                             Thickness (μm)                                                                      2   2   1.4 2    2    2    2    2    2    2                          A10.sup.4 (dyne/cm.sup.2)                                                              6000                                                                              7000                                                                              6000                                                                              6000 6000 15000                                                                              5800 1200 7200 7800                       A10 (dyne/cm.sup.2)                                                                    900 800 900 900  900  900  900  500  800  900                        A10.sup.4 /A10                                                                         6.67                                                                              8.75                                                                              6.67                                                                              6.67 6.67 150.00                                                                             6.44 2.40 9.00 8.6                        Coating System                                                                         wet-on-                                                                           wet-on-                                                                           wet-on-                                                                           wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                             wet wet wet wet  dry  wet  wet  wet  wet  wet                        Evaluation:                                                                   7 MHz Output (dB)                                                                      5.5 5.1 4.1 0.5  5.2  0.2  4.7  N.D.**                                                                             5.5  5.1                        Running  good to                                                                           good                                                                              good to                                                                           good --   good bad  --   good good                       Durability                                                                             medium  medium                                                       Yield (%)                                                                              99  99.5                                                                              99  98   2    0.5  97   0.3  99   99.5                       __________________________________________________________________________     Note:                                                                         *: Average primary particle diameter                                          **: Nondetectable                                                        

As is apparent from the results in Table 20, Samples 12-1 through 12-5falling within means (B) according to the present invention exhibitedexcellent results in all the items of 7 MHz output, running durability,and yield of production.

To the contrary, Comparative Sample 12'-1 in which layer (b) has athickness greater than 1 μm suffered from a reduction in 7 MHz output.Comparative Sample 12'-2 which was prepared by wet-on-drying coating hada very poor yield. Comparative Samples 12'-3 and 12'-5 having a A10⁴/A10 ratio out of the range of the present invention both had a lowyield and underwent a reduction in 7 MHz output. Further, ComparativeSample 12'-4 in which layer (a) contained no polyisocyanate hadconsiderably reduced running durability.

EXAMPLE 13

Means (C) according to the present invention was examined. Coatingcompositions for layers (a) and (b) used in sample preparation wereprepared as follows.

    ______________________________________                                        Layer (a):                                                                    Co-Doped iron oxide (Hc:                                                                            0 or 100  parts                                         9500 Oe; BET specific surface                                                 area: 58 m.sup.2 /g; crystallite                                              size: 250 Å; particle size                                                (major axis): 0.20 μm; aspect                                              ratio: 8)                                                                     Vinyl chloride-vinyl acetate-vinyl                                                                  13        parts                                         alcohol copolymer (polar group                                                (-N(CH.sub.3).sub.3.sup.+ Cl.sup.-1) content: 5 × 10.sup.-6 eq/g)       Polyester polyurethane resin                                                                        11        parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            α-Iron oxide (acicular; major                                                                 see Table 3                                             axis: 0.3 μm; BET specific                                                 surface area: 45 m.sup.2 /g; pH: 3.5)                                         α-Alumina (particle size: 0.3 μm)                                                          5         parts                                         Carbon black (average primary                                                                       100 or 500                                                                              parts                                         particle diameter: 16 mμ; DBP                                              absorption: 80 ml/100 g; pH: 8.0;                                             BET specific surface area: 250 m.sup.2 /g;                                    volatile content: 1.5%)                                                       Butyl stearate        1         part                                          Stearic acid          2         parts                                         Methyl ethyl ketone   400       parts                                         ______________________________________                                        Layer (b):                                                                    Ferromagnetic metal fine powder                                                                     100       parts                                         (Fe/Zn/Ni = 92/4/4; Hc: 1600 Oe;                                              BET specific surface area: 60 m.sup.2 /g;                                     crystallite size: 195 Å; particle                                         size (major axis): 0.20 μm;                                                aspect ratio: 10; σ.sub.2 : 130 emu/g)                                  Vinyl chloride copolymer (-SO.sub.3 Na                                                              12        parts                                         content: 1 × 10.sup.-4 eq/g; degree of                                  polymerization: 300)                                                          Polyester polyurethane resin                                                                        3         parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            α-Alumina (particle size: 0.3 μm)                                                          2         parts                                         Carbon black (particle size: 0.10 μm)                                                            0.5       part                                          Butyl stearate        1         part                                          Stearic acid          2         parts                                         Methyl ethyl ketone   200       parts                                         ______________________________________                                    

The above components for each of layers (a) and (b) were kneaded in acontinuous kneader and then dispersed in a sand mill. To the resultingdispersions for layer (a) and layer (b) was added 6 parts and 3 parts ofpolyisocyanate, respectively, and 40 parts of butyl acetate was furtheradded to each of the dispersions. The dispersion was filtered through afilter having an average pore size of 1 μm to prepare a coatingcomposition for layer (a) and a coating composition for layer (b). TheA10⁴ /A10 ratio of these coating compositions is shown in Table 21below.

Preparation of Samples 13-1 to 13-6 and Comparative Samples 13'-1 to13'-7 and 13'-9

A polyethylene terephthalate film having a thickness of 7 μm and acenterline surface roughness (Ra) of 0.01 μm was coated with the coatingcomposition for layer (a) to a dry thickness of 2 μm and immediatelythereafter with the coating composition for layer (b) to a dry thicknessof 0.3 μm by wet-on-wet coating to form layers (a) and (b). While bothlayers were wet, the ferromagnetic powder in layer (b) was orientated byapplying a magnetic field using cobalt magnet having a magnetic force of3000 G and a solenoid having a magnetic force of 1500 G. After drying,the coated film was calendered through 7 metallic rolls at 90° C. andcut to a width of 8 mm to prepare 8 mm-video tape.

In Comparative Sample 13'-4, layers (a) and (b) were formed bywet-on-dry coating as follows. The coating composition for layer (a) wascoated to a dry thickness of 2.0 μm, dried, rolled, and calendered. Thethus formed layer (a) was subjected to a curing treatment at 80° C. for24 hours, and the coating composition for layer (b) was then coatedthereon to a dry thickness of 0.3 μm, subjected to orientation, dried,rolled up, and calendered in the same manner as described above.

Preparation of Sample 13-7 and Comparative Sample 13'-8

Samples were prepared in the same manner as for Sample 13-1, except forusing the following Co-doped γ-Fe₂ O₃ as a ferromagnetic powder in layer(b).

Co-Doped γ-Fe₂ O₃

Hc: 1400 Oe

BET Specific Surface Area: 45 m² /g

Crystallite Size: 290 Å

Particle Size (major axis): 0.3 μm

Aspect Ratio: 10

σ_(s) : 75 emu/g

Each of Samples 13-1 to 13-7 and Comparative Samples 13'-1 to 13'-9 wasevaluated in the same manner as in Example 12. The results obtained areshown in Table 21.

                                      TABLE 21                                    __________________________________________________________________________                                             Compara.                                                                           Compara.                                    Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                            Sample                                                                             Sample                                      13-1 13-2 13-3 13-4 13-5 13-6                                                                              13'-1                                                                              13'-2                           __________________________________________________________________________    Layer (b):                                                                    Hc (Oe)     1600 1000 1600 1600 1250 2900                                                                              1600 1000                            Brn (gauss) 3500 3500 3500 3500 3500 3500                                                                              3500 3500                            Thickness (μm)                                                                         0.3  0.3  0.3  0.8  0.3  0.3 1    0.3                             Ferromagnetic Powder                                                                      A*   A*   A*   A*   A*   A*  A*   A*                              Layer (a):                                                                    Magnetic Iron Oxide (part)                                                                100  100  100  100  100  100 100  100                             α-Iron Oxide (part)                                                                 600  600  250  600  600  600 600  600                             Carbon Black (part)                                                                       100  100  100  100  100  100 100  100                             HC (Oe)     950  950  950  950  950  950 950  950                             Bm (gauss)  320  320  480  320  320  320 320  320                             A10.sup.4 (dyne/cm.sup.2)                                                                 3000 3000 4000 3000 3000 3000                                                                              3000 3000                            A10 (dyne/cm.sup.2)                                                                       250  250  400  250  250  250 250  250                             A10.sup.4 /A10                                                                            12   12   10   12   12   12  12   12                              Coating System                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                           wet-on-                                                                            wet-on-                                     wet  wet  wet  wet  wet  wet wet  wet                             Evaluation:                                                                   7 MHZ Output (dB)                                                                         6.6  7    6.5  5.3  6    7.3'                                                                              0.5  2.2                             Yield (%)   98   95   96   97   99   100 99   98                              __________________________________________________________________________                Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara. Compara.                                                                           Compara.                                    Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                            Sample                                                                             Sample                                      13'-3                                                                              13'-4                                                                              13'-5                                                                              13'-6                                                                              13'-7                                                                              13-7                                                                              13'-8                                                                              13'-9                           __________________________________________________________________________    Layer (b):                                                                    Hc (Oe)     1600 1600 1600 3300 1150 1500                                                                              1500 1600                            Bm (gauss)  3500 3500 3500 3500 3500 2100                                                                              1800 4600                            Thickness (μm)                                                                         0.3  0.3  0.3  0.3  0.3  0.3 0.3  0.3                             Ferromagnetic Powder                                                                      A*   A*   A*   A*   A*   B** B**  A*                              Layer (a):                                                                    Magnetic Iron Oxide (part)                                                                0    100  100  100  100  100 100  100                             α-Iron Oxide (part)                                                                 4000 600  200  650  600  600 600  600                             Carbon Black (part)                                                                       500  100  100  100  100  100 100  100                             HC (Oe)     950  950  950  950  950  950 950  950                             Bm (gauss)  0    320  560  320  320  320 320  320                             A10.sup.4 (dyne/cm.sup.2)                                                                 11000                                                                              3000 4000 3000 3000 3000                                                                              3000 3000                            A10 (dyne/cm.sup.2)                                                                       30   250  1500 250  250  250 250  250                             A10.sup.4 /A10                                                                            366.7                                                                              12   2.7  12   12   12  12   12                              Coating System                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                           wet-on-                                                                            wet-on-                                     wet  dry  wet  wet  wet  wet wet  wet                             Evaluation:                                                                   7 MHZ Output (dB)                                                                         6.5  6.6  0.5  2.1  1.3  5.5 2.1  -1.5                            Yield (%)   25   11   99   99   98   98  99   99                              __________________________________________________________________________     Note:                                                                         A*: Fe--Zn--Ni alloy                                                          B**: CoDoped Fe.sub.2 O.sub.3                                            

It can be seen from the results in Table 21 that Samples 13-1 through13-6 according to means (C) of the present invention exhibited animproved output at 7 MHz and an improved yield whereas ComparativeSamples 13'-1 to 13'-9 which are out of the scope of the presentinvention failed to obtain sufficient results in 7 MHz output or yield.

As demonstrated by the foregoing Examples, the present inventionconveniently provides a magnetic recording medium having a smallmagnetic layer thickness while exhibiting excellent durability andelectromagnetic characteristics by preparing coating compositions forlayers (a) and (b) having controlled thixotropy by adjusting thephysical properties of magnetic or non-magnetic powders, thecomposition, etc., particularly of layer (a), as described herein, andcoating the magnetic and non-magnetic coating compositions by wet-on-wetcoating, either simultaneously or successively.

EXAMPLE 14

Means (D) according to the present invention was examined. Coatingcompositions for layers (a) and (b) used for sample preparation wereprepared as follows.

    ______________________________________                                        Layer (a):                                                                    Non-magnetic powder α-Fe.sub.2 O.sub.3                                                           80     parts                                         (TF-100 produced by Toda Kogyo                                                K.K.; average particle size:                                                  0.1 μm; BET specific surface                                               area: 11 m.sup.2 /g; pH: 5.6)                                                 Carbon black (average primary                                                                          20     parts                                         particle diameter: 16 nm; DBP                                                 absorption: 80 ml/100 g; pH:                                                  8.0; BET specific surface area:                                               250 m.sup.2 /g; volatile content: 1.5%)                                       Vinyl chloride-vinyl acetate-vinyl                                                                     10     parts                                         alcohol (86:13:1) copolymer (polar                                            group (-N(CH.sub.3).sub.3.sup.+ Cl.sup.-1) content:                           5 × 10.sup.-6 eq/g; degree of                                           polymerization: 400)                                                          Polyester polyurethane resin                                                                           8      parts                                         (basic skeleton: 1,4-BD/phthalic                                              acid/HMDI; molecular weight: 10200;                                           OH group: 0.23 × 10.sup.-3 eq/g; -SO.sub.3 Na                           group: 1 × 10.sup.-4 eq/g)                                              Butyl stearate           1      part                                          Stearic acid             1      part                                          Methyl ethyl ketone      200    parts                                         ______________________________________                                        Layer (b):                                                                    Ferromagnetic metal fine powder                                                                        100    parts                                         (Fe/Zn/Ni = 92/4/4; Hc: 1600 Oe;                                              BET specific surface area: 60 m.sup.2 /g;                                     crystallite size: 200 Å; particle                                         size (major axis): 0.20 μm;                                                aspect ratio: 10; σ.sub.2 : 130 emu/g)                                  Vinyl chloride copolymer (-SO.sub.3 Na                                                                 12     parts                                         content: 1 × 10.sup.-4 eq/g; degree of                                  polymerization: 300)                                                          Polyester polyurethane resin                                                                           3      parts                                         (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content: 1 × 10.sup.-4 eq/g)                                            α-Alumina (particle size: 0.3 μm)                                                             2      parts                                         Carbon black (particle size: 0.10 μm)                                                               0.5    part                                          Butyl stearate           1      part                                          Stearic acid             2      parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components for each of layers (a) and (b) were kneaded in acontinuous kneader and then dispersed in a sand mill. To the resultingdispersions for layer (a) and layer (b) was added 5 parts and 6 parts ofpolyisocyanate, respectively, and 40 parts of butyl acetate was furtheradded to each of the dispersions. The dispersion was filtered through afilter having an average pore size of 1 μm to prepare a coatingcomposition for layer (a) and a coating composition for layer (b).

Preparation of Sample 14-1

A polyethylene terephthalate film having a thickness of 7 μm and acenterline surface roughness (Ra) of 0.01 μm was coated with the coatingcomposition for layer (a) to a dry thickness of 2 μm and immediatelythereafter with the coating composition for layer (b) to a dry thicknessof 0.2 μm by wet-on-wet coating to form layers (a) and (b). While bothlayers were wet, the ferromagnetic powder in layer (b) was orientated byapplying a magnetic field using cobalt magnet of 3000 G and a solenoidof 1500 G. After drying, the coated film was calendered through 7metallic rolls at 90° C. and cut to a width of 8 mm to prepare 8mm-video tape.

Preparation of Samples 14-2 to 14-6 and Comparative Samples 14'1 to14'-5

Video tape samples were prepared in the same manner as for Sample 14-1,except for changing the amounts of carbon black and α-Fe₂ O₃ in layer(a) (Samples 14-2 and 14-3 and Comparative Samples 14'-1 and 14'-2),decreasing the amount of a hydroxyl group in polyurethane (ComparativeSample 14'-3), changing the particle size of carbon black (ComparativeSample 14'-4), or changing the thickness of layer (b) (Samples 14-4 to14-6 and Comparative Sample 14'-5). These alterations made are shown inTable 22 below. Further, in Sample 14-4, the following ferromagneticpowder Fe/Zn/Ni was used.

Fe/Zn/Ni=92/4/4

Hc: 15800 Oe

BET Specific Surface Area: 42 m² /g

Crystallite Size: 280 Å

Particle Size (major axis): 0.30 μm

Aspect Ratio: 10

σ_(s) : 140 emu/g

Preparation of Comparative Sample 14'-6

A coating composition for layer (b) was prepared as described above,except for increasing the amount of methyl ethyl ketone to 700 parts todecrease the concentration. The same support as used in Sample 14-1 wascoated with the coating composition for layer (a). After performing athorough curing reaction of layer (a), the above-prepared dilutedcoating composition for layer (b) was then coated thereon to a drythickness of 0.3 μm, orientated, dried, and calendered to obtain 8mm-video tape.

Preparation of Comparative Sample 14'-7

A sample was prepared in the same manner as for Sample 14-1, except forusing the following ferromagnetic powder in layer (b).

Fe/Zn/Ni=92/4/4

Hc: 15800 Oe

BET Specific Surface Area: 35 m² /g

Crystallite Size: 330 Å

Particle Size (major axis): 0.35 μm

Aspect Ratio: 10

σ_(s) : 140 emu/g

Preparation of Sample 14-7

A video tape sample was prepared in the same manner as for Sample 14-1,except for using the following composition as a coating composition forlayer (b).

    ______________________________________                                        Co-Doped iron oxide (Hc: 950 Oe;                                                                      100    parts                                          BET specific surface area: 58 m.sup.2 /g;                                     crystallite size: 250 Å; particle                                         size (major axis): 0.20 μm; aspect                                         ratio: 8)                                                                     Vinyl chloride-vinyl acetate-vinyl                                                                    13     parts                                          alcohol (86:13:1) copolymer (polar                                            group (-N(CH.sub.3).sub.3.sup.+ Cl.sup.-1) content:                           5 × 10.sup.-6 eq/g; degree of                                           polymerization: 400)                                                          Polyester polyurethane resin                                                                          3      parts                                          (neopentyl glycol/caprolactone                                                polyol/MDI = 0.9/2.6/1; -SO.sub.3 Na                                          content:. 1 × 10.sup.-4 eq/g)                                           α-Alumina (particle size: 0.3 μm)                                                            2      parts                                          Carbon black (particle size: 100 μm)                                                               0.5    part                                           Butyl stearate          1      part                                           Stearic acid            2      parts                                          Methyl ethyl ketone     200    parts                                          ______________________________________                                    

Preparation of Sample 14-8

A sample was prepared in the same manner as for Sample 14-1, except forusing the following ferromagnetic powder in layer (b).

Co-doped barium ferrite

Hc: 1800 Oe

BET Specific Surface Area: 35 m² /g

Average Particle Size: 0.05 μm

Aspect Ratio: 5

σ_(s) : 68 emu/g

Performance properties of Samples 14-1 to 14-8 and Comparative Samples14'-1 to 14'-7 were evaluated according to the following test methods.The results obtained are shown in Table 22.

Coating Properties

The coated layers were observed while in an undried state. A sample inwhich mixing of layers (a) and (b) at the interface did not occur wasrated "good", and a sample in which such mixing took place was rated"bad". Since those samples suffering such mixing of layers were notprocessed any further, evaluations of 7 MHz output and scratchresistance hereinafter described were not made with respect to thosesamples.

7 MHz Output

Evaluated in the same manner as in Example 11.

Scratch Resistance

The magnetic layer of a sample was scratched with a stainless steelstylus under a load of 15 g. The damage on the magnetic layer wasvisually observed with the naked eye and rated "good", "medium", and"bad" in the descending order.

Magnetic Flux Density

A magnetic flux density was measured with a vibrating samplemagnetometer (manufactured by Toei Kogyo K.K.) at Hm of 5 kOe.

Major Axis of Magnetic Powder

An average particle diameter of the major axis was obtained by means ofa transmission type electron microscope.

Crystallite Size

Calculated from the half-value width of the X-ray diffraction line ofthe (4,4,0) face and (2,2,0) face.

                                      TABLE 22                                    __________________________________________________________________________                           Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                           Sample                                                                            Sample                                                                            Sample                                                                            Sample                                                                             Sample                                                                             Sample                                                                             Example                                                                            Sample                                        14-1                                                                              14-2                                                                              14-3                                                                              14'-1                                                                              14'-2                                                                              14'-3                                                                              14'-4                                                                              14-4                               __________________________________________________________________________    Metallic Powder                                                                          metal                                                                             metal                                                                             metal                                                                             metal                                                                              metal                                                                              metal                                                                              metal                                                                              metal                              Layer (b):                                                                    Major axis of Magnetic                                                                   0.2 0.2 0.2 0.2  0.2  0.2  0.2  0.3                                Powder (μm)                                                                Crystallite Size of                                                                      200 200 200 200  200  200  200  280                                Magnetic Powder (Å)                                                       Thickness (μm)                                                                        0.3 0.3 0.3 0.3  0.3  0.3  0.3  0.3                                Layer (a):                                                                    Particle Size of                                                                         16  16  16  16   16   16   25   16                                 Carbon Black (μm)                                                          Carbon Black/α-Fe.sub.2 O.sub.3                                                    20/80                                                                             5/95                                                                              40/60                                                                             3/97 50/50                                                                              20/80                                                                              20/80                                                                              20/80                              OH/molecule-Urethane                                                                     3   3   3   3    3    2    3    3                                  Coating System                                                                           wet-on-                                                                           wet-on-                                                                           wet-on-                                                                           wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                       wet wet wet wet  wet  wet  wet  wet                                Evaluations:                                                                  Coating Properties                                                                       good                                                                              good                                                                              good                                                                              bad  good bad  bad  good                               7 MHz Output (dB)                                                                        7   5   6.1 --   -2   --   --   4                                  Scratch Resistance                                                                       good                                                                              good                                                                              good                                                                              --   good --   --   good                               __________________________________________________________________________                         Compara.                                                                           Compara.                                                                            Compara.                                                  Sample                                                                            Sample                                                                             Sample                                                                             Sample                                                                              Sample                                                                             Sample                                                                              Sample                                         14-5                                                                              14-6 14'-5                                                                              14'-6 14'-7                                                                              14-7  14-8                               __________________________________________________________________________    Metallic Powder                                                                           metal                                                                             metal                                                                              metal                                                                              metal metal                                                                              Co-doped                                                                            BaFe                                                                    γ-Fe.sub.2 O.sub.3                 Layer (b):                                                                    Major axis of Magnetic                                                                    0.2 0.2  0.2  0.2   0.35 0.25  0.05                               Powder (μm)                                                                Crystallite Size of                                                                       200 200  200  200   330  280                                      Magnetic Powder (Å)                                                       Thickness (μm)                                                                         0.5 1    1.3  0.3   0.3  0.3   0.3                                Layer (a):                                                                    Particle Size of                                                                          16  16   16   16    16   16    16                                 Carbon Black (μm)                                                          Carbon Black/α-Fe.sub.2 O.sub.3                                                     20/80                                                                             20/80                                                                              20/80                                                                              20/80 20/80                                                                              20/80 20/80                              OH/molecule-Urethane                                                                      3   3    3    3     3    3     3                                  Coating System                                                                            wet-on-                                                                           wet-on-                                                                            wet-on-                                                                            wet-on-                                                                             wet-on-                                                                            wet-on-                                                                             wet-on-                                        wet wet  wet  dry   wet  wet   wet                                Evaluations:                                                                  Coating Properties                                                                        good                                                                              good good --    bad  good  good                               7 MHZ Output (dB)                                                                         6   1    -1.0 -1.5  --   3     3                                  Scratch Resistance                                                                        good                                                                              good good bad   --   good  good                               __________________________________________________________________________

As is apparent from the results in Table 22, the samples according tothe present invention can be prepared with satisfactory coatingproperties without involving mixing of the lower and upper layers, andthey exhibited a high output of signals 7 MHz and satisfactory scratchresistance. To the contrary, Comparative Sample 14'-1 in which thecarbon black to non-magnetic powder ratio is out of the scope of thepresent invention had insufficient coating properties and underwent areduction in output. Comparative Samples 14'-3 and 14'-4 in which thehydroxyl group content in the polyurethane was too small or the particlesize of carbon black was too large exhibited unsatisfactory coatingproperties. Further, where the major axis or crystallite size of theferromagnetic powder was too large as in Comparative Sample 14'-7, thecoating properties were also deteriorated.

EXAMPLE 15

Means (E) and (F) according to the present invention were examined asfollows.

Preparation of Sample 15-1

    ______________________________________                                        Layer (a):                                                                    ______________________________________                                        Acicular α-Fe.sub.2 O.sub.3 (major axis:                                                         100    parts                                         0.5 μm; aspect ratio: 10)                                                  Carbon black (average particle                                                                         5      parts                                         size: 20 μm)                                                               Vinyl chloride copolymer (containing                                                                   8      parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na,                                                           5      parts                                         molecular weight: 45000)                                                      Cyclohexane              100    parts                                         Methyl ethyl ketone      100    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 4hours. To the dispersion were added polyisocyanate (Colonate L), 5 partsof stearic acid, and 10 parts of butyl stearate to prepare a coatingcomposition for layer (a).

    ______________________________________                                        Layer (b):                                                                    ______________________________________                                        Ferromagnetic powder: Fe/Ni/Co alloy                                                                   100    parts                                         (92/6/2; Hc: 1600 Oe; σ.sub.s : 135 emu/g                               major axis: 0.18 μm; aspect ratio: 9)                                      Vinyl chloride copolymer (containing                                                                   10     parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na,                                                           5      parts                                         molecular weight: 45000)                                                      α-Alumina (average particle size: 0.2 μm)                                                     5      parts                                         Cyclohexanone            150    parts                                         Methyl ethyl ketone      150    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 6hours. To the dispersion were added 5 parts of stearic acid and 10 partsof butyl stearate to prepare a coating composition of layer (b).

A support was coated with the above-prepared coating compositions forlayers (a) and (b) to a dry thickness of 3.0 μm and 0.3 μm,respectively, by wet-on-wet coating using two doctor blades differing ingap size. The magnetic powder was orientated in a magnetic field, thecoated layers were dried and calendered. The resulting film was cut to awidth of 3.81 mm to prepare digital audio tape (DAT).

Preparation of Samples 15-2 to 15-7 and Comprartive Samples 15'-1 to15'-6

Samples were prepared in the same manner as for Sample 15-1, except formaking changes as shown in Table 23 below.

In Samples 15-6 and 15-7, the following barium ferrite was used as aferromagnetic powder.

Ba Ferrite

Hc: 1100 Oe

σ_(s) : 70 emu/g

Plate Diameter: 0.05 μm

Aspect Ratio: 5

Each of Samples 15-1 to 15-7 and Comparative Samples 15'-1 to 15'-6 wasevaluated according to the following test methods. The results obtainedare shown in Table 23.

Mixed Region

A sample tape was sandwiched in between epoxy resin articles, cooled inliquid nitrogen, and sliced along the lengthwise and width directions bymeans of a microtome. The cut surface was observed under a transmissiontype electron microscope (TEM) at a magnifying power of 50,000.Observations were made as follows.

1) When the shape of particles differs between layer (a) and layer (b),for example, the ferromagnetic powder in layer (b) is acicular with thenon-magnetic powder in layer (a) being particulate or flaky, or theferromagnetic powder in layer (b) is tabular with the non-magneticpowder in layer (a) being particular or acicular, whether there is amixed region or not was judged from the degree of mixing of particles ofdifferent shape.

2) When the particles of layers (a) and (b) have the same shape butdiffer in average length of the maximum major axis, for example, theferromagnetic powder in layer (b) and the non-magnetic powder in layer(a) both have an acicular shape but are different in the above-describedaverage diameter, or the ferromagnetic powder in layer (b) is tabularwith the non-magnetic powder in layer (a) being flaky, whether there isa mixed region or not was judged from the degree of mixing of particlesof different diameter.

3) When the particles of layers (a) and (b) are equal to each other inboth particle shape and average diameter, whether there is a mixedregion or not was judged by detecting elements inherent to each layer inthe vicinity of the interface between layers (a) and (b) according to amicroauger electron spectroscopy.

The dry thickness of layer (b) of each sample was measured under TEMobservation of a specimen prepared in the same manner as describedabove.

Output on Reproduction (RF Output)

Signals of constant frequency 4.7 MHz were input, and reproduced signalswere recorded by means of a spectrum analyzer HP-3585A. The peak valueof the signals was read out and relatively expressed taking the resultsof Comparative Example 15'-1 as a standard (0 dB).

Block Error Rate (BER)

A block error rate is a number of error flags per 10000 tracks. ##EQU4##

DAT has a signal processing system of coding analog signals into digitalsignals. One signal comprises 8 bits, and one block comprises 32signals×8 pits=256 bits. Accordingly, one track is constituted by 128blocks.

Signals of two tracks, i.e., 128×2 blocks, were put in memory andshuffled, and the errors were detected. A DAT deck "DTC-1000" producedby Sony Corporation was used, and a counter "HP5328A" produced byHewlett Packard Co. was connected to a standard personal computer.

Drop Out (DO)

Signals of a single frequency 4.7 MHz were input, and drop outs of athreshold level -10 dB and a length of 0.5 μsec were counted with a dropout counter.

                                      TABLE 23                                    __________________________________________________________________________                 Sample                                                                              Sample                                                                             Sample                                                                              Sample                                                                              Sample                                                                             Sample                                            15-1  15-2 15-3  15-4  15-5 15-6                                 __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder                                                                       Fe alloy                                                                            Fe alloy                                                                           Fe alloy                                                                            Fe alloy                                                                            Fe alloy                                                                           barium                                                                        ferrite                              Major axis (μm)                                                                         0.18  0.18 0.18  0.18  0.18                                      Plate Diameter (μm)                                                                     --    --   --    --    --   0.1                                  Thickness (μm)                                                                          0.3   0.3  0.85  0.15  0.3  0.3                                  Layer (a):                                                                    Non-magnetic Powder                                                                        αFe.sub.2 O.sub.3                                                             αFe.sub.2 O.sub.3                                                            αFe.sub.2 O.sub.3                                                             αFe.sub.2 O.sub.3                                                             αAl.sub.2 O.sub.3                                                            αAl.sub.2 O.sub.3              Aspect Ratio 10    10   10    10    8    8                                    Major axis (μm)                                                                         0.5   0.5  0.5   0.5   0.3  0.3                                  Average Particle Size (μm)                                                              --    --   --    --    --   --                                   Thickness (μm)                                                                          3     0.8  2.5   3.5   3    3                                    Coating System                                                                             wet-on-                                                                             wet-on-                                                                            wet-on-                                                                             wet-on-                                                                             wet-on-                                                                            wet-on-                                           wet   wt   wet   wet   wet  wet                                  Mixed Region not   not  not   not   not  not                                               observed                                                                            observed                                                                           observed                                                                            observed                                                                            observed                                                                           observed                             RF Output (dB)                                                                             4.5   2.9  3.2   3.6   4.2  3.7                                  Drop Out     60    70   45    180   60   100                                  BER          7 × 10.sup.-6                                                                 4 × 10.sup.-5                                                                4 × 10.sup.-5                                                                 3 × 10.sup.-5                                                                 1 × 10.sup.-5                                                                8 × 10.sup.-6                  __________________________________________________________________________                     Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                                                           Compara.                                        Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                          15-7 15'-1                                                                              15'-2                                                                              15'-3                                                                              15'-4                                                                              15'-5                                                                              15'-6                               __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder                                                                      barium                                                                             Fe   Fe   Fe   Fe   barium                                                                             Fe                                              ferrite                                                                            alloy                                                                              alloy                                                                              alloy                                                                              alloy                                                                              ferrite                                                                            alloy                               Major axis (μm)                                                                        --   0.18 0.18 0.18 0.18 0.18 0.18                                Plate Diameter (μm)                                                                    0.1  --   --   --   --   --                                       Thickness (μm)                                                                         0.3  3.3  0.3  0.3  0.3  1.2  0.3                                 Layer (a):                                                                    Non-magnetic Powder                                                                       acicular                                                                           --   αFe.sub.2 O.sub.3                                                            parti-                                                                             parti-                                                                             αFe.sub.2 O.sub.3                                                            αFe.sub.2 O.sub.3                         Ti             culate                                                                             culate                                                                   alumina                                                                            alumina                                       Aspect Ratio                                                                              15   --   10   --   --   10   10                                  Major axis (μm)                                                                        1.2  --   0.5  --   --   0.5  0.5                                 Average Particle Size (μm)                                                             --   --   --   0.3  0.3  --   --                                  Thickness (μm)                                                                         3    --   3    3    3    3    0.3                                 Coating System                                                                            wet-on-                                                                            single                                                                             wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                                                            wet-on-                                         wet  layer                                                                              dry  wet  dry  wet  wet                                 Mixed Region                                                                              not  --   observed                                                                           observed                                                                           observed                                                                           observed                                                                           observed                                        observed                                                          RF Output (dB)                                                                            3.1  0    3.8  -1.5 3.3  0.8  -0.8                                Drop Out    80   150  1200 500  2000 75   320                                 BER         3 × 10.sup.-5                                                                2 × 10.sup.-4                                                                5 × 10.sup.-4                                                                8 × 10.sup.-4                                                                6 × 10.sup.-4                                                                1 × 10.sup.-4                                                                3 × 10.sup.-4                 __________________________________________________________________________

EXAMPLE 16

Means (G) according to the present invention was examined as follows.

    ______________________________________                                        Layer (a):                                                                    ______________________________________                                        Graphite (particle size: 0.5 μm)                                                                    90     parts                                         α-Al.sub.2 O.sub.3 (average particle size: 0.2 μm)                                            10     parts                                         Vinyl chloride copolymer (epoxy                                                                        15     parts                                         content: 8 × 10.sup.-4 eq/g; molecular                                  weight: 45000)                                                                Polyurethane (containing -SO.sub.3 Na;                                                                 5      parts                                         molecular weight: 45000)                                                      Cyclohexanone            100    parts                                         Methyl ethyl ketone      100    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 4hours. To the dispersion were added polyisocyanate (Colonate L), 5 partsof stearic acid, and 10 parts of butyl stearate to prepare a coatingcomposition of layer (a).

    ______________________________________                                        Layer (b):                                                                    ______________________________________                                        Ferromagnetic powder: Fe-Ni-Co                                                                         100    parts                                         (92:6:2; Hc: 1600 Oe; σ.sub.s : 135 emu/g;                              major axis: 0.18 μm; aspect ratio: 9)                                      Vinyl chloride copolymer (containing                                                                   10     parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na;                                                           5      parts                                         molecular weight: 45000)                                                      α-Alumina (average particle size: 0.2 μm)                                                     5      parts                                         Cyclohexanone            150    parts                                         Methyl ethyl ketone      150    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 6hours, and to the dispersion were added polyisocyanate (Colonate L), 5parts of stearic acid, and 10 parts of butyl stearate to obtain acoating composition of layer (b).

A support was coated with the above-prepared coating compositions forlayers (a) and (b) to a dry thickness of 3.0 μm and 0.3 μm,respectively, by wet-on-wet coating using two doctor blades differing ingap size. The magnetic powder was orientated in a magnetic field usingpermanent magnet, the coated layers were dried and calendered. Theresulting film was cut to a width of 3.81 mm to prepare digital audiotape (DAT). The dry thickness of layer (b) was measured in the samemanner as in Example 15.

Preparation of Samples 16-2 to 16-9 and Comparative Samples 16'-1 To16'-9

Samples were prepared in the same manner as for Sample 16-1, except formaking alterations as show in Table 24 below. Barium ferrite used as amagnetic powder had the following properties.

Barium Ferrite

Hc: 1100 Oe

σ_(s) : 70 emu/g

Plate Diameter: 0.05 μm

Aspect Ratio: 5

Each of Samples 16-1 to 16-9 and Comparative Samples 16'-1 to 16'-9 wasevaluated in the same manner as in Example 15. The 4.7 MHz output wasexpressed relatively taking the result of Comparative Sample 16'-1 as astandard (0 dB). In addition, an increase in drop out was measuredaccording to the following test method. The results obtained are shownin Table 24.

DO Increase

In the measurement of drop out, the sample was played 100 passes on thesame deck at 23° C. and 70% RH, and then reproduced. An increase in DOover the initial DO was obtained.

                                      TABLE 24                                    __________________________________________________________________________              Sample                                                                              Sample                                                                              Sample                                                                              Sample                                                                              Sample                                                                              Sample                                          16-1  16-2  16-3  16-4  16-5  16-6                                  __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder                                                                    Fe alloy                                                                            Fe alloy                                                                            Fe alloy                                                                            Fe alloy                                                                            Fe alloy                                                                            Fe alloy                              Major axis (μm)                                                                      0.18  0.18  0.18  0.18  0.18  0.18                                  Plate Diameter (μm)                                                                  --    --    --    --    --    --                                    Thickness (μm)                                                                       0.3   0.3   0.3   0.3   0.3   0.15                                  Layer (a):                                                                    Non-magnetic Powder                                                                     graphite                                                                            graphite                                                                            graphite                                                                            graphite                                                                            graphite                                                                            graphite                              Plate Diameter (μm)                                                                  0.5   0.5   0.5   0.2   2     0.5                                   Abrasive  α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            Cr.sub.2 O.sub.3                      Particle Size (μm)                                                                   0.2   0.2   0.2   0.5   0.2   0.5                                   Non-magnetic Powder/                                                                    90/10 95/5  60/40 80/20 80/20 90/10                                 Abrasive Ratio                                                                Binder:                                                                       Epoxy Content (eq/g)                                                                    8 × 10.sup.-4                                                                 8 × 10.sup.-4                                                                 8 × 10.sup.-4                                                                 8 × 10.sup.-4                                                                 16 × 10.sup.-4                                                                8 × 10.sup.-4                   Molecular weight                                                                        45000 45000 45000 55000 35000 45000                                 Thickness (μm)                                                                       3     3     3     3     2.5   3.5                                   Coating System                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                            Mixed Region                                                                            not   not   not   not   not   not                                             observed                                                                            observed                                                                            observed                                                                            observed                                                                            observed                                                                            observed                              RF Output (dB)                                                                          4.5   4.2   3.8   4.6   3.2   3.6                                   Drop Out  60    30    80    70    45    90                                    BER (× 10.sup.-6)                                                                 7.0   5.0   5.5   8.0   20    30                                    Drop Out Increase                                                                       30    11    13    28    45    30                                    __________________________________________________________________________                                Compara.                                                                            Compara.                                                                            Compara.                                        Sample                                                                              Sample                                                                              Sample                                                                              Sample                                                                              Sample                                                                              Sample                                          16-7  16-8  16-9  16'-1 16'-2 16'-3                                 __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder                                                                    Fe alloy                                                                            Fe alloy                                                                            Ba ferrite                                                                          Fe alloy                                                                            Fe alloy                                                                            Fe alloy                              Major axis (μm)                                                                      0.18  0.18  --    0.18  0.18  0.18                                  Plate Diameter (μm)                                                                  ----        0.1   --    --    --                                    Thickness (μm)                                                                       0.3   0.3   0.3   3.3   0.3   0.3                                   Layer (a):                                                                    Non-magnetic Powder                                                                     boron mica  graphite                                                                            --    graphite                                                                            carbon                                          nitride                       black                                 Plate Diameter (μm)                                                                  0.5   0.5   0.5   --    0.5   0.2                                   Abrasive  α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            α-Fe.sub.2 O.sub.3                                                            --    α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3              Particle Size (μm)                                                                   0.2   0.2   0.5   --    0.2   0.5                                   Non-magnetic Powder/                                                                    90/10 90/10 90/10 --    90/10 90/10                                 Abrasive Ratio                                                                Binder:                                                                       Epoxy Content (eq/g)                                                                    8 × 10.sup.-4                                                                 8 × 10.sup.-4                                                                 8 × 10.sup.-4                                                                 --    8 × 10.sup.-4                                                                 8 × 10.sup.-4                   Molecular Weight                                                                        45000 45000 45000 --    45000 45000                                 Thickness (μm)                                                                       3     0.8   3     --    3     3                                     Coating System                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          single                                                                              wet-on-dry                                                                          wet-on-wet                                                        layer                                             Mixed Region                                                                            not   not   not   --    observed                                                                            observed                                        observed                                                                            observed                                                                            observed                                                RF Output (dB)                                                                          4.2   3.7   3.9   0     3.8   -1.5                                  Drop Out  60    100   45    150   1200  500                                   BER (× 10.sup.-6)                                                                 10    8.0   9.0   400   900   1200                                  Drop Out Increase                                                                       14    26    33    75    110   46                                    __________________________________________________________________________              Compara.                                                                            Compara.                                                                            Compara.                                                                            Compara.                                                                            Compara.                                                                            Compara.                                        Sample                                                                              Sample                                                                              Sample                                                                              Sample                                                                              Sample                                                                              Sample                                          16'-4 16'-5 16'-6 16'-7 16'-8 16-'9                                 __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder                                                                    Fe alloy                                                                            Fe alloy                                                                            Fe alloy                                                                            Fe alloy                                                                            Fe alloy                                                                            Fe alloy                              Major axis (μm)                                                                      0.18  0.18  0.18  0.18  0.18  0.4                                   Plate Diameter (μm)                                                                  --    --    --    --    --    --                                    Thickness (μm)                                                                       0.3   1.2   0.3   0.3   0.3   0.3                                   Layer (a):                                                                    Non-magnetic Powder                                                                     graphite                                                                            graphite                                                                            graphite                                                                            graphite                                                                            graphite                                                                            graphite                              Plate Diameter (μm)                                                                  0.5   0.5   0.5   0.5   0.5   0.5                                   Abrasive  none  α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3                                                            α-Al.sub.2 O.sub.3              Particle Size (μm)                                                                   --    0.2   0.2   0.2   0.2   0.2                                   Non-magnetic Powder/                                                                    --    90/10 50/50 90/10 90/10 90/10                                 Abrasive Ratio                                                                Binder:                                                                       Epoxy Content (eq/g)                                                                    none  none  8 × 10.sup.-4                                                                 8 × 10.sup.-4                                                                 8 × 10.sup.-4                                                                 8 × 10.sup.-4                   Molecular Weight                                                                        --    45000 45000 45000 25000 45000                                 Thickness (μm)                                                                       3     3     3     0.3   3     3                                     Coating System                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet                                                                          wet-on-wet-                           Mixed Region                                                                            observed                                                                            observed                                                                            observed                                                                            observed                                                                            observed                                                                            observed                              RF Output (dB)                                                                          4     -3    -1.2  -0.8  -1.8  -0.5                                  Drop Out  48    75    320   320   160   56                                    BER (× 10.sup.-6)                                                                 un-   2000  900   300   1100  100                                             measurable                                                          Drop Out Increase                                                                       2500  230   8     18    78    21                                    __________________________________________________________________________

As is apparent from the results in Table 23, it is demonstrated thatSamples 15-1 through 15-7, in which an acicular magnetic powder is used,have an improved RF output, a reduced drop out, and a low BER. In Table24, it was also proved that Samples 16-1 to 16-9, in which a flakynon-magnetic powder is used, have improved durability (indicated by theinhibited increase in drop out).

To the contrary, Comparative Samples 15'-1 to 15'-6 and 16'-1 to 16'-9,which are out of the scope of the present invention, failed to attainsatisfactory results in one or more of BER, drop out, and RF output. Forassistance in understanding the data, it is noted that desired levels ofBER, drop out, and RF output are tyipically not more than 10⁻⁴, not morethan 10, and not less than 3.0 dB, respectively.

As described above, the present invention provides a magnetic recordingmedium having a plurality of layers formed by a wet-on-wet coatingsystem in which an upper magnetic layer is coated simultaneously orsuccessively while a lower non-magnetic layer is wet, in which thelayers do not undergo mixing together at the interface. That is, themagnetic recording material according to the present invention exhibitssatisfactory electromagnetic characteristics, particularly a high RFoutput, excellent running durability, a reduced drop out, and a lowblock error rate and can be produced with good productivity.

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Particulate TiO.sub.2 (average particle                                                                100    parts                                         size: 0.04 μm)                                                             Carbon black (average particle size:                                                                   5      parts                                         20 mμ)                                                                     Vinyl chloride copolymer (containing                                                                   8      parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na;                                                           5      parts                                         molecular weight: 45000)                                                      α-Alumina (average particle size: 0.2 μm)                                                     5      parts                                         Cyclohexanone            150    parts                                         Methyl ethyl ketone      50     parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 4hours, and to the dispersion were added 5 parts of polyisocyanate(Coronate L), 5 parts of stearic acid, and 10 parts of butyl stearate toprepare a coating composition of layer (a).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: 5% Co-doped Fe                                                                   100    parts                                         alloy (Hc: 1600 Oe; σ.sub.s : 135 emu/g;                                major axis length: 0.18 μm; acicular                                       ratio: 9)                                                                     Vinyl chloride copolymer (containing                                                                   10     parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na;                                                           5      parts                                         molecular weight: 45000)                                                      α-Alumina (average particle size: 0.2 μm)                                                     5      parts                                         Cyclohexanone            100    parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 6hours, and to the dispersion were added 5 parts of polyisocyanate(Coronate L), 5 parts of stearic acid, and 10 parts of butyl stearate toprepare a coating composition of layer (b).

The coating compositions for layers (a) and (b) were simultaneouslycoated on a 5.5 μm thick polyoxyethylene naphthalate film support bywet-on-wet coating using two doctor blades set at different gaps to adry thickness of 2.5 μm (layer a) and 0.5 μm (layer b). While the layers(a) and (b) were wet, the ferromagnetic powder in layer (b) wasorientated with a permanent magnet, followed by drying. A back layercontaining carbon black was coated on the opposite side of the support.Thereafter, the coated film was subjected to supercalendering. Theresulting magnetic recording medium was cut to a width of 3.81 mm toobtain a digital audio tape (DAT). This sample was designated 17-1.

Samples 17-2 to 17-8 and Comparative Samples 17'-1 to 17'-8 wereprepared in the same manner as for Sample 17-1, except for makingalterations according to Tables 25 or 26 below.

Each sample was evaluated according to the following test methods.

Thickness (d) of Magnetic Layer

Measured according to a fluorescent X-ray method. A calibration curve offluorescent X-ray intensity for an element inherently contained in amagnetic layer is prepared from magnetic layer samples having a knownthickness, and a thickness of a sample of unknown thickness is obtainedfrom its fluorescent X-ray intensity.

Residual Coercive Force (Hr)

A magnetic field of 10 kOe was applied to the sample in the in-planedirection of layer (b). When the thus magnetized sample was turned 90°in the thickness direction, the intensity of the outer magnetic fieldapplied in the normal direction with respect to layer (b) which wasrequired for making the residual magnetization zero was-measured.

In the following testing of electromagnetic characteristics, a DAT deck"DTC-1000" produced by Sony Corporation was used.

Reproduction Output (RF Output)

Signals of single frequency 4.7 MHz were recorded, and the reproducedsignals were put into a spectrum analyzer, "HP-3585A". The peak of thesignals was read out. The result of Comparative Sample 17'-7 was takenas a standard (0 dB).

Block Error Rate (BER)

A block error rate is a number of error flags per 10000 tracks. ##EQU5##

DAT has a signal processing system of coding analog signals into digitalsignals. One signal comprises 8 bits, and one block comprises 32signals×8 pits=256 bits. Accordingly, one track is constituted by 128blocks.

Signals of two tracks, i.e., 128×2 blocks, were put in memory andshuffled, and the errors were detected. A DAT deck "DTC-1000" producedby Sony Corporation was used, and a counter "HP5328A" produced byHewlett Pockard Co. was connected to a standard personal computer.

Dropout (DO)

Signals of a single frequency 4.7 MHz were input, and dropouts 0.5 μsecin length were counted with a dropout counter on a threshold level of-10 dB.

                                      TABLE 25                                    __________________________________________________________________________               Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                     17-1 17-2 17-3 17-4 17-5 17-6 17-7 17-8                            __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder:                                                         Kind       Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Ba ferrite                                                                         Ba ferrite                      Hc (Oe)    1600 1540 1580 1600 1600 1620 1300 1300                            Major axis length (μm)                                                                0.18 0.2  0.16 0.18 0.18 0.25 --   --                              Plate diameter (μm)                   0.1  0.1                             Hr (Oe)    1800 1640 1750 1800 1620 1760 1800 1800                            d (μm)  0.5  0.5  0.5  0.8  0.5  0.5  0.5  0.5                             Layer (a):                                                                    Inorganic Powder:                                                             Kind       TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          α-Fe.sub.2 O.sub.3        True specific gravity                                                                    4.2  4.2  4.2  4.2  4.2  4.2  4.2  5.0                             Average particle                                                                         0.04 0.04 0.04 0.04 0.12 0.04 0.04 --                              size (μm)                                                                  Acicular ratio                                12                              Major axis length (μm)                     0.3                             Thickness (μm)                                                                        2.5  2.5  2.5  2.5  2.5  2.5  2.5  2.5                             Evaluation:                                                                   RF Output (dB)                                                                           4.5  3.2  4.2  3.9  4    3    3.5  3.8                             Drop out   60   50   35   55   75   40   68   70                              BER        7 × 10.sup.-6                                                                2 × 10.sup.-5                                                                9 × 10.sup.-6                                                                1 × 10.sup.-5                                                                3 × 10.sup.-5                                                                6 × 10.sup.-5                                                                4 × 10.sup.-5                                                                4 × 10.sup.-5             __________________________________________________________________________

                                      TABLE 26                                    __________________________________________________________________________               Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                    Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                     17'-1                                                                              17'-2                                                                              17'-3                                                                              17'-4                                                                              17'-5                                                                              17'-6                                                                              17'-7                                                                              17'-8                           __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder:                                                         Kind       Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Ba ferrite                                                                         Ba ferrite                      Hc (Oe)    1250 1250 1600 1600 1600 1600 1600 1600                            Major axis length (μm)                                                                0.3  0.3  0.18 0.18 0.18 0.18 0.4  0.18                            Hr (Oe)    1300 1300 1450 1390 1690 1690 1700 1470                            d (μm)  0.5  0.5  0.5  1.5  0.5  0.5  0.5  0.5                             Layer (a):                                                                    Inorganic Powder:                                                             Kind       TiO.sub.2 TiO.sub.2                                                                          αFe.sub.2 O.sub.3                                                            TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                                          carbon                                                                        black                           True specific gravity                                                                    4.2       4.2  5.0  4.2  4.2  4.2  2                               Average particle                                                                         0.04      0.3       0.04 0.04 0.04 0.02                            size (μm)                                                                  Acicular ratio                                                                           --        --   12   --   --   --   --                              Major axis length (μm) 1.5                                                 Thickness (μm)                                                                        2.5  2.5  2.5  2.5  2.5  2.5  2.5  2.5                             Evaluation:                                                                   RF Output (dB)                                                                           0.7  -2.8 0.5  1    -0.3 0.2  0    0.5                             Dropout    120  1800 100  65   45   1500 160  150                             BER        3 × 10.sup.-3                                                                all  5 × 10.sup.-3                                                                2 × 10.sup.-3                                                                8 × 10.sup.-3                                                                5 × 10.sup.-2                                                                6 × 10.sup.-3                                                                9 × 10.sup.-4                             error                                                         __________________________________________________________________________

As shown in Tables 25 and 26, Samples 17-1 to 17-8 according to thepresent invention exhibit satisfactory electromagnetic characteristics,i.e., high RF output, low dropout, and low BER. Comparative Samples17'-1 to 17'-4 and 17'-8 have low Hr and show no improvement in output.Comparative Sample 17'-2 using no inorganic powder in layer (a) is poorin dropout and BER. Comparative Sample 17'-4 using an inorganic powderof high acicular ratio achieves no improvement in electromagneticcharacteristics assumably because of disturbed orientation of theferromagnetic alloy powder in layer (b). Both of Comparative Sample17'-5 having a thick magnetic layer (1.5 μm) and Comparative Sample17'-6 having a thin non-magnetic lower layer (0.3 μm ) did not exhibitsatisfactory characteristics. Comparative Sample 17'-7 using aferromagnetic layer of long major axis (0.4 μm) also failed to obtainimproved characteristics.

EXAMPLE 18

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Particulate TiO.sub.2 (average particle                                                                100    parts                                         size: 0.04 μm)                                                             Carbon black (average particle size:                                                                   5      parts                                         20 mμ)                                                                     Vinyl chloride copolymer (containing                                                                   8      parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na;                                                           5      parts                                         molecular weight: 45000)                                                      α-Alumina (average particle size: 0.2 μm)                                                     5      parts                                         Cyclohexanone            150    parts                                         Methyl ethyl ketone      50     parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 4hours, and to the dispersion were added 5 parts of polyisocyanate(Coronate L), 5 parts of stearic acid, and 10 parts of butyl stearate toprepare a coating composition of layer (a).

    ______________________________________                                        Coating Composition for Layer (b):                                            ______________________________________                                        Ferromagnetic powder: 5% Co-doped Fe                                                                   100    parts                                         alloy (Hc: 1600 Oe; σ.sub.s : 130 emu/g;                                major axis length: 0.18 μm; acicular                                       ratio: 8)                                                                     Vinyl chloride copolymer (containing                                                                   10     parts                                         -SO.sub.3 Na and epoxy group)                                                 Polyurethane resin (containing -SO.sub.3 Na;                                                           5      parts                                         molecular weight: 45000)                                                      α-Alumina (average particle size: 0.2 μm)                                                     5      parts                                         Cyclohexanone            100    parts                                         Methyl ethyl ketone      200    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 6hours, and to the dispersion were added 5 parts of polyisocyanate(Coronate L), 5 parts of stearic acid, and 10 parts of butyl stearate toprepare a coating composition of layer (b).

The coating compositions for layers (a) and (b) were simultaneouslycoated on a 5.5 μm thick polyoxyethylene naphthalate film support bywet-on-wet coating using two doctor blades set at different gaps to adry thickness of 2.5 μm (layer a) and 0.5 μm (layer b). While the layers(a) and (b) were wet, the ferromagnetic powder in layer (b) wasorientated with a permanent magnet, followed by drying. A back layercontaining carbon black was coated on the opposite side of the support.Thereafter, the coated film was subjected to supercalendering. Theresulting magnetic recording medium was cut to a width of 3.81 mm toobtain a digital audio tape (DAT). This sample was designated 18-1.

Samples 18-2 to 18-5 and Comparative Samples 18'-1 to 18'-6 wereprepared in the same manner as for Sample 18-1, except for makingalterations according to Tables 27 or 28 below. Comparative Sample 18'-2was prepared by successive wet-on-dry coating.

Each sample was evaluated in the same manner as in Example 17, and theresults obtained are shown in Tables 27 and 28.

                                      TABLE 27                                    __________________________________________________________________________                Sample                                                                             Sample                                                                              Sample                                                                             Sample                                                                              Sample                                                  18-1 18-2  18-3 18-4  18-5                                        __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder:                                                         Kind        Fe alloy                                                                           Fe alloy                                                                            Fe alloy                                                                           Fe alloy                                                                            Fe alloy                                    Major axis length (μm)                                                                 0.18 0.24  0.18 0.18  0.18                                        Acicular ratio                                                                            8    10    11   8     8                                           Hc in MD (Oe)                                                                             1600 1550  1700 1600  1600                                        Hc in TD (Oe)                                                                             1200 1100  1180 1070  1230                                        d (μm)   0.5  0.5   0.85 0.15  0.3                                         Layer (a):                                                                    Inorganic Powder:                                                             Kind        TiO.sub.2                                                                          TiO.sub.2                                                                           TiO.sub.2                                                                          αFe.sub.2 O.sub.3                                                             αFe.sub.2 O.sub.3                     True specific gravity                                                                     4.2  4.2   4.2  5     5                                           Average particle                                                                          0.04 0.04  0.04                                                   size (μm)                                                                  Acicular ratio              7     16                                          Major axis length (μm)   0.3   0.8                                         Thickness (μm)                                                                         2.5  0.8   2.5  3.5   3                                           Evaluation:                                                                   RF Output (dB)                                                                            4.5  2.9   3.2  3.6   4.2                                         Dropout     60   70    45   180   60                                          BER         2 × 10.sup.-6                                                                8 × 10.sup.-5                                                                 2 × 10.sup.-6                                                                1 × 10.sup.-5                                                                 7 × 10.sup.-6                         __________________________________________________________________________

                                      TABLE 28                                    __________________________________________________________________________               Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                                                            Compar.                                              Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                                                             Sample                                               18'-1                                                                              18'-2                                                                              18'-3                                                                              18'-4                                                                              18'-5                                                                              18'-6                                     __________________________________________________________________________    Layer (b):                                                                    Ferromagnetic Powder:                                                         Kind       Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                                                           Fe alloy                                  Major axis length (μm)                                                                0.18 0.18 0.18 0.3  0.24 0.18                                      Acicular ratio                                                                           8    8    8    10   19   8                                         Hc in MD (Oe)                                                                            1600 1600 1600 1250 1650 1600                                      Hc in TD (Oe)                                                                            1040 1150 1060 870  900  1200                                      d (μm)  3.3  0.5  0.5  0.5  0.5  0.5                                       Layer (a):                                                                    Inorganic Powder:                                                             Kind            TiO.sub.2                                                                          carbon                                                                             TiO.sub.2                                                                          TiO.sub.2                                                                          TiO.sub.2                                                      black                                                    True specific gravity                                                                         4.2  2    4.2  4.2  4.2                                       Average particle                                                                              0.04 0.02 0.04 0.04 0.04                                      size (μm)                                                                  Thickness (μm)                                                                             2.5  2.5  2.5  2.5  0.3                                       Evaluation:                                                                   RF Output (dB)                                                                           0    3.8  -1.5 -1.8 0.8  -0.8                                      Dropout    150  1200 50   80   75   750                                       BER        2 × 10.sup.-4                                                                5 × 10.sup.-4                                                                3 × 10.sup.-3                                                                2 × 10.sup.-3                                                                1 × 10.sup.-4                                                                8 × 10.sup.-4                       __________________________________________________________________________

As can be seen from Tables 27 and 28, the samples according to thepresent invention exhibit excellent electromagnetic characteristics,i.e., high RF output, reduced dropout, and reduced BER. ComparativeSample 18'-1 is an example having no lower non-magnetic layer.Comparative Sample 18'-2, which is prepared by wet-on-dry coating systemhas increased BER and increased dropout. Comparative Sample 18'-3 usingcarbon black in place of an inorganic powder does not satisfy BER andreproduction output. Comparative Sample 18'-4 using an excessively largeferromagnetic alloy powder has a low Hc in both TD and MD and thereforefails to obtain high reproduction output and also suffers from high BER.Comparative Sample 18'-5 using a ferromagnetic powder of high acicularratio has a lower Hc in TD than the level specified in the presentinvention and therefore fails to obtain high reproduction output andalso shows no improvement in BER. Comparative Sample 18'-6 having a thinlower layer fails to satisfy all the electromagnetic requirements.

EXAMPLE 19

A polyethylene terephthalate film support (thickness: 10 μm; F5 value:20 kg/mm² in MD, 14 kg/mm² in TD; Young's modulus: 750 kg/mm² in MD, 470kg/mm² in TD) (hereinafter abbreviated as PET support) or a polyethyleneterenaphthalate film support (thickness: 7 μm; F5 value: 22 kg/mm² inMD, 18 kg/mm² in TD; Young's modulus: 750 kg/mm² in MD, 750 kg/mm² inTD) (hereinafter abbreviated as PEN support) was used.

Coating Composition for Subbing Layer

    ______________________________________                                        --SO.sub.3 Na-Containing polyester                                                                100       parts                                           resin (Tg: 65° C.; Na content:                                         4600 ppm)                                                                     Cyclohexanone       9900      parts                                           ______________________________________                                    

The above composition was stirred in a disper stirrer for 12 hours, andthe resulting coating composition was coated on the PET or PEN supportwith a bar coater to a dry thickness of 0.1 μm.

    ______________________________________                                        Coating Composition for Layer (a):                                            ______________________________________                                        Rutile TiO.sub.2 (average particle size:                                                               85     parts                                         0.035 μm; TiO.sub.2 content: ≧90%; surface                          treating agent: Al.sub.2 O.sub.3 ; BET specific                               surface area: 35 to 45 m.sup.2 /g; true                                       specific gravity: 4.1; pH: 6.5 to 8.0)                                        Carbon black (average particle size:                                                                   5      parts                                         16 mμ; DBP absorption: 80 ml/100 g;                                        pH: 8.0; BET specific surface area:                                           250 m.sup.2 /g; coloring power: 143%)                                         Vinyl chloride copolymer (-SO.sub.3 Na                                                                 13     parts                                         content: 8 × 10.sup.-5 eq/g; containing -OH                             and epoxy groups; Tg: 71° C.; polymer-                                 ization degree: 300; Mn: 12000; Mw:                                           38000)                                                                        Polyurethane resin (-SO.sub.3 Na content:                                                              5      parts                                         8 × 10.sup.-5 eq/g; -OH content: 8 × 10.sup.-5                    eq/g; Tg: 38° C.; Mw: 50000)                                           Cyclohexane              100    parts                                         Methyl ethyl ketone      100    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 4hours. To the mixture were added 5 parts of polyisocyanate (Coronate L),5 parts of oleic acid, 5 parts of stearic acid, and 15 parts of butylstearate to prepare a coating composition for layer (a).

    ______________________________________                                        Coating Composition of Layer (b):                                             ______________________________________                                        Ferromagnetic powder: Fe/Co/Ni alloy                                                                   100    parts                                         (92:6:2; sintering inhibitor: Al.sub.2 O.sub.3 ;                              Hc: 1600 Oe; σ.sub.s : 119 emu/g; major                                 axis length: 0.13 μm; acicular ratio:                                      7; crystallite size: 172 Å; water                                         content: 0.6%)                                                                Vinyl chloride copolymer (-SO.sub.3 Na                                                                 13     parts                                         content: 8 × 10.sup.-5 eq/g; containing                                 -OH and epoxy group; Tg: 71° C.; poly-                                 merization degree: 300; number average                                        molecular weight (Mn): 12000; weight                                          average molecular weight (Mw): 38000)                                         Polyurethane resin (-SO.sub.3 Na content:                                                              5      parts                                         8 × 10.sup.-5 eq/g; -OH content: 8 × 10.sup.-5                    eq/g; Tg: 38° C.; Mw: 50000)                                           α-Alumina (average particle size:                                                                12     parts                                         0.15 μm; BET specific surface area:                                        8.7 m.sup.2 /g; pH: 8.2; water content:                                       0.06%)                                                                        Cyclohexanone            150    parts                                         Methyl ethyl ketone      150    parts                                         ______________________________________                                    

The above components were mixed and dispersed in a sand mill for 6hours. To the mixture were added 5 parts of polyisocyanate (Coronate L),5 part of oleic acid, 7 parts of stearic acid, and 15 parts of butylstearate to prepare a coating composition for layer (b).

On the subbing layer side of the support were simultaneously coated withthe coating compositions for layers (a) and (b) by use of two doctorblades set at different gaps to form layer (a) having a dry thickness of3.0 μm and layer (b) having a dry thickness of 0.3 μm. The ferromagneticpowder in layer (b) was orientated with a permanent magnet of 3500 G anda solenoid of 1600 G. After drying, the coated film was supercalenderedthrough metallic rolls at 80° C.

    ______________________________________                                        Coating Composition for Back Layer:                                           ______________________________________                                        Carbon black (BET specific surface                                                                     100    parts                                         area: 220 m.sup.2 /g; average particle                                        size: 17 mμ; DBP absorption: 75 ml/100 g;                                  volatile content: 1.5%; pH: 8.0; bulk                                         density: 15 lbs/ft.sup.3)                                                     Nitrocellulose "RS1/2"   100    parts                                         Polyester polyurethane "Nippollan"                                                                     30     parts                                         Dispersing agent:                                                               Copper oleate          10     parts                                           Copper phthalocyanine  10     parts                                           Barium sulfate (precipitated)                                                                        5      parts                                         Methyl ethyl ketone      500    parts                                         Toluene                  500    parts                                         ______________________________________                                    

The above components were preliminarily kneaded and then kneaded in aroll mill. To 100 parts of the resulting dispersion were added 100 partsof carbon black (BET specific surface area: 200 m² /g; average particlesize: 200 mμ; DBP absorption: 36 ml/100 g; pH: 8.5) and 0.1 part ofα-Al₂ O₃ (average particle size: 0.2 μm), and the mixture was dispersedin a sand grinder, followed by filtration. To 100 parts of the resultingdispersion were further added 120 parts of methyl ethyl ketone and 5parts of polyisocyanate to prepare a coating composition for a backlayer.

The resulting coating composition was coated on the non-magnetic supporton the side opposite to layer (b) with a bar coater to a dry thicknessof 0.5 μm. The resulting magnetic recording medium was cut to a width of8 mm to prepare a 8 mm video tape. The sample using a PET support wasdesignated sample 11-1, and that using a PEN support Sample 11-2.

Each of Samples 19-1 and 19-2 was evaluated according to the followingtest methods.

1) TEM Observations

A sample was sliced with a diamond cutter to prepare an about 0.1 μmthick specimen. The specimen was photographed with TEM. The interfacebetween layers (a) and (b) and the surface of layer (b) were marked, andthe thickness of layer (b) was measured with an image analyzer "IBAS II"to obtain an average d and a standard deviation σ.

As a result, layer (b) had a d of 0.45 μm. A layer (b) thicknesssuitable for practical use was proved to be not more than 1 μm, andparticularly not more than 0.6 μm. The standard deviation σ of the layer(b) thickness variation was found to be not more than 0.08 μm. Apractically useful σ was proved to be not more than 0.02 μm, andparticularly not more than 0.01 μm.

The above-prepared sample was stretched to have layer (b) released fromthe support, and layer (b) was scraped off with a cutter blade. The thusremoved layer (b) weighing 500 mg was refluxed in 100 ml of a 1NNaOH/methanol solution for 2 hours to hydrolyze the binders. Thesupernatant liquor was removed with the ferromagnetic powder of greaterspecific gravity being precipitated. The solid was washed three timeswith water and then three times with tetrahydrofuran, followed by dryingin a vacuum drier at 50° C. The resulting ferromagnetic powder wasdispersed in collodion and observed under TEM (×60000). Theferromagnetic powder was found to have a major axis length of 0.13 μmand an acicular ratio of 10. It was proved that a major axis lengthshould be not more than 0.4 μm, and preferably not more than 0.3 μm, forpractical use and that an acicular ratio should fall within a range offrom 2 to 20, and preferably from 2 to 15, for practical use.

2) Atomic Force Microscope (AFM)

Surface roughness R_(rms) was obtained by scanning the surface of layer(b) with "Nanoscope II" manufactured by Digital Instrument Co. over anarea of 6 μm×6 μm at a tunnel current of 10 nA and a bias voltage of 400mV.

As a result, R_(rms) was 6 nm. It was proved that R_(rms) should be notmore than 10 nm, and preferably not more than 8 nm, for practical use.

3) Surface Roughness Tester

Surface roughness was measured using 3d-MIRAU. Centerline surfaceroughness (Ra), R_(rms), and peak-to-valley value of layer (b) over anarea of about 250×250 nm were measured according to a MIRAU method bymeans of "TOPO 3D" manufactured by WYKO K.K. Spherical corrections andcylindrical corrections were made at a measuring wavelength of about 650nm. This testing system is a non-contact roughness tester utilizinginterference of light.

As a result, Ra was 2.7 nm. It was proved that a practically useful Rais from 1 to 4 nm, and particularly from 2 to 3.5 nm. R_(rms) was 3.5nm. A practically useful R_(rms) is from 1.3 to 6 nm, and particularlyfrom 1.5 to 5 nm. P-V value was between 20 and 30 nm. A practicallyuseful P-V value is not more than 80 nm, and particularly from 10 to 60nm.

4) Vibrating Sample Magnetometer (VSM)

Magnetic characteristics of the sample were measured at Hm of 5 kOe withVSM manufactured by Toei Kogyo K.K.

The result were Hc: 1620 Oe; Hr: 1800 Oe; Br/Bm: 0.82; SFD: 0.583. Forpractical use, it was found that Hc should be from 1500 to 2500 Oe, andpreferably from 1600 to 2000 Oe; Hr should be from 1000 to 2800 Oe, andpreferably from 1200 to 2500 Oe; Br/Bm should be at least 0.75, andpreferably at least 0.8; and SFD should be 0.7 or less, and preferably0.6 or less.

5) X-Ray Diffraction

X-ray diffratometry was conducted using the ferromagnetic powder sampledfrom layer (b) in 1) above. The magnetic tape was directly set on anX-ray diffractometer. A crystallite size of the ferromagnetic powder wasobtained from the half-width value of the diffraction pattern of thefaces (4,4,0) and (2,2,0). As a result, the crystallite size was 180 Å.For practical use, a crystallite size is preferably not more than 400 Å,and more preferably from 100 to 300 Å.

6) Tensile Test

Young's modulus, yield stress, and yield elongation of the magnetic tapewere measured with a tensile tester "STM-T-50BP" manufactured by ToyoBaldwin K.K. in an atmosphere of 23° C. and 70% RH at a rate of pullingof 10%/min.

As a result, the sample had a Young's modulus (modulus of elasticity at0.5% elongation) of kg/mm², a yield stress of from 6 to 7 kg/mm², and ayield elongation of 0.8%. For practical use, a Young's modulus (modulusof elasticity at 0.5% elongation) preferably ranges from 400 to 2000kg/mm², and more preferably from 500 to 1500 kg/mm² ; a yield stresspreferably ranges from 3 to 20 kg/mm², and more preferably from 4 to 15kg/mm² ; a yield elongation preferably ranges from 0.2 to 8%, and morepreferably from 0.4 to 5%.

7) Stiffness in Flexure, Loop Stiffness

Stiffness in flexure was expressed in terms of a force (mg) required forgiving a 5 mm displacement to a 8 mm wide and 50 mm long sample in aloop form with use of a loop stiffness tester at a rate of displacementof about 3.5 mm/sec.

As a result, the 8 mm wide p6-120 tape having a thickness of 10.5 μm hada stiffness between 40 and 60 mg. With a thickness of 10.5±1 μm, apreferred stiffness is from 20 to 90 mg, and particularly from 30 to 70mg, for practical use. With a thickness of 11.5 μm or more, a preferredstiffness is from 40 to 200 mg. With a thickness of 9.5 μm or less, apreferred stiffness is from 10 to 70 mg.

8) Elongation at Failure

Elongation at cracking was measured at 23° C. and 70% RH. A 10 cm longspecimen was pulled at both ends thereof at a rate of 0.1 mm/sec whilemicroscopically observing the surface of layer (b). The elongation (%)at which 5 or more clear cracks developed on the surface of layer (b)was measured.

As a result, the sample had an elongation at cracking of 4%. It wasproved that a preferred elongation at cracking is not more than 20%, andparticularly not more than 10%, for practical use.

9) Percent Thermal Shrinkage

The sample was preserved in a thermostat at 70° C. for 48 hours. Thechange in length between before and after the preservation was dividedby the length before preservation to obtain a percent thermal shrinkage.

As a result, the percent thermal shrinkage of the sample was 0.2%. Itwas proved that a practically preferred percent thermal shrinkage is notmore than 0.4%, and particularly between 0.1 and 0.3%.

10) ESCA

Cl/Fe spectrum α and N/Fe spectrum β were measured with an X-rayphotoelectric spectrophotometer (manufactured by Perkin-Elmer Co.) at300 W using an Mg anode as an X-ray source. After washing away thelubricant in the sample with n-hexane, the sample was set in an X-rayphotoelectric spectrophotometer at a distance of 1 cm from the X-raysource. After 5 minutes from evacuation, Cl-2P spectrum, N-1S spectrum,and Fe-2P (3/2) spectrum were integrated for 10 minutes. A pass energywas fixed at 100 eV. An integrated intensity ratio of the Cl-2P spectrumto the Fe-2P (3/2) spectrum was calculated to obtain α. An integratedintensity ratio of the N-1S spectrum to the Fe-2P (3/2) spectrum wascalculated to obtain β.

As a result, α was 0.45, and β was 0.07. It was proved that apractically preferred range of α is from 0.3 to 0.6, and particularlyfrom 0.4 to 0.5, and that of β is from 0.03 to 0.12, and particularlyfrom 0.04 to 0.1.

11) Dynamic Viscoelastometer

Dynamic viscoelasticity of the sample was measured at 110 Hz with adynamic viscoelastometer "Rheovibron" manufactured by Toyo Baldwin Co.The peak temperature at E" was taken as Tg. This measurement systemcomprises adding vibration to one end of the tape and measuring thevibration transmitted to the other end.

It was found, as a result, that Tg was 73° C.; E' (50° C.) was 4×10¹⁰dyne/cm² ; and E" (50° C.) was 1×10¹¹. It was proved that a practicallypreferred range of Tg is from 40 to 120° C., and particularly from 50 to110° C., that of E' (50° C.) is from 0.8×10¹¹ to 11×10¹¹ dyne/cm², andparticularly from 1×10¹¹ to 9×10¹¹ dyne/cm², and that of E" (50° C.) isfrom 0.5×10¹¹ to 8×10¹¹ dyne/cm², and particularly from 0.7×10¹¹ to5×10¹¹ dyne/cm².

12) Adhesive Strength

Adhesive tape produced by 3M was adhered onto a 8 mm wide sample, and a180° peel strength between the support and the magnetic layer wasmeasured at 23° C. and 70% RH.

As a result, the adhesive strength was 50 g. It was proved that anadhesive strength is preferably 10 g or more, and particularly 20 g ormore, for practical use.

13) Wearability

The sample was placed on a slide glass with both ends thereof fixed withadhesive tape, and a steel ball 6.25 mm in diameter was slid thereonunder a load of 50 g. In this case, the ball was once slid over adistance of 20 mm at a speed of 20 mm/sec and then moved to a freshmagnetic layer surface, where the same sliding was repeated 20 times.Thereafter, the sliding surface of the steel ball was observed with amicroscope (×40) to obtain its diameter, assuming the sliding surfacebeing a circle. The abrasion wear was calculated from the measureddiameter.

As a result, the abrasion wear was found to be 0.7×10⁻⁵ to 1.1×10⁻⁵ mm³.For practical use, the abrasion wear was from 0.1×10⁻⁵ to 5×10⁻⁵ mm³,and particularly from 0.4×10⁻⁵ to 2×10⁻⁵ mm³.

14) Scanning Electron Microscope (SEM)

Five micrographs were taken of the surface of layer (b) with SEM "S-900"manufactured by Hitachi, Ltd. (×5000). The average number of abrasivegrains appearing on the surface was found to be 0.2/μm². It was provedthat a practically usually number of abrasive grains is at least0.1/μm², and particularly from 0.12 to 0.5/μm².

15) Gas Chromatography (GC)

A specimen having an area of 20 cm² was heated to 120° C., and theresidual solvent was measured with a gas chromatograph "GC-14A"manufactured by Shimazu Seisakusho Ltd.

As a result, the residual solvent was 8 mg/m². It was found that aresidual solvent is preferably not more than 50 mg/m², and particularlynot more than 20 mg/m², for practical use.

16) Sol Fraction

A weight ratio of tetrahydrofuran-soluble solid contents of the magneticlayer to the magnetic layer was found to be 7%. It was proved that a solfraction is preferably not more than 15%, and particularly not more than10%, for practical use.

Samples 19-1 and 19-2 were compared with commercially available 8 mmvideo tapes according to the above-mentioned test methods or commonlyemployed methods, and the results obtained are shown in Table 29.Standards for rating the results were as follows.

Jitter

Good . . . less than 0.2 μsec

Bad . . . 0.2 μsec or more

Preservation Stability

Good . . . No rust occurred after preservation at 60° C. and 90% RH for2 weeks.

Bad . . . Rust occurred after preservation at 60° C. and 90% RH for 2weeks.

Running Durability

Good . . . No jamming occurred during 100 passes.

Bad . . . Jamming occurred during 100 passes.

Scratch Resistance

Good . . . No scratches were visually perceived after 10 minutes runningin a still mode.

Bad . . . Scratches were visually perceived after 10 minutes running ina still mode.

                  TABLE 29                                                        ______________________________________                                                                 Single                                                        Sample Sample   Coated-   Deposited                                           19-1   19-2     Metal Tape.sup.1)                                                                       Tape.sup.2)                                ______________________________________                                        Electromagnetic                                                               Characteristics:                                                              7 MHz Output (dB)                                                                        5.5      6.0      3.0     6.2                                      C/N (dB)   4.3      4.5      2.0     4.1                                      Color S/N (dB)                                                                           2.5      2.6      2.5     -3.0                                     Video S/N (dB)                                                                           2.1      2.3      1.5     0.5                                      Durability:                                                                   Dropout    40       30       30      580                                      BER (×10.sup.-5)                                                                   4        2        50      80                                       Jitter     good     good     good    bad                                      Still      ≧30 min                                                                         ≧30 min                                                                         ≧30 min                                                                        ≧30 min                           Head Wear  1.2      1.4      2.0     0.2                                      (μm/100 hr)                                                                Preservation                                                                             good     good     good    bad                                      Stability                                                                     (60° C., 90% RH)                                                       Running    good     good     good    bad                                      Durability                                                                    Scratch    good     good     good    medium                                   Resistance                           to bad                                   ______________________________________                                         Note:                                                                         .sup.1) : Product of Fuji Photo Film Co., Ltd.; Lot No. 407209M               .sup.2) : Product of Sony Corporation; Lot No. 709011CD                  

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A magnetic recording medium, comprising:asupport; at least two layers comprising a lower non-magnetic layerdisposed at one side of the support, the lower non-magnetic layercomprising a non-magnetic powder and a binder, and an upper magneticlayer disposed over the lower non-magnetic layer, the upper magneticlayer comprising a ferromagnetic powder and a binder; and a back layerat the other side of the support, the back layer comprising carbon blackand a binder, wherein the upper magnetic layer has an average drythickness (d) of 1.0 μm or less; the non-magnetic powder contained inthe lower non-magnetic layer is a non-magnetic inorganic powder having aMohs hardness of 3 or more; and the back layer has a surface free energyof 10 to 100 dyne/cm.
 2. The magnetic recording medium as claimed inclaim 1, wherein the ferromagnetic powder contained in the uppermagnetic layer is a ferromagnetic alloy powder having an average longestaxis of 0.3 μm or less.
 3. The magnetic recording medium as claimed inclaim 1, wherein the carbon black contained in the back layer comprisescarbon black having an average particle size shorter than an averagelongest axis of the ferromagnetic powder and carbon black having anaverage particle size longer than the average longest axis of theferromagnetic powder.
 4. The magnetic recording medium as claimed inclaim 1, wherein the back layer has a μ of 0.15 to 0.4.
 5. The magneticrecording medium as claimed in claim 1, which has an average thicknessvariation (Ad) at the interface between the upper magnetic layer andlower non-magnetic layer after dying is 0.001 to 0.5 μm.
 6. The magneticrecording medium as claimed in claim 1, wherein the back layer has asurface resistivity of not more than 1×10⁹ Ω/sq.
 7. The magneticrecording medium as claimed in claim 1, wherein the lower non-magneticlayer comprises carbon black having an average particle size of 5 to 80nm.
 8. The magnetic recording medium as claimed in 1, wherein the uppermagnetic layer has a surface having an average number of abrasive grainsappearing of at least 0.1/μm², when five micrographs of the magneticrecording medium are taken at the surface with SEM with a magnificationof 50,000.
 9. The magnetic recording medium as claimed in claim 1,wherein the binder contained in the back layer comprises polyurethaneand nitrocellulose.